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Mouth and Stomach Brooders: can they be saved from extinction?

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Written by Roger Downie, Froglife and University of Glasgow

On 3rd February 2025, many media outlets reported news of the ‘birth’ of 33 juvenile Darwin’s frogs at London Zoo. Why was this event so newsworthy? In this article, we look into two unusual modes of parental care in frogs and the plight of the species involved.

Among the more than 70 reproductive modes so far described from amphibians (see Croaking Science December 2021 and April 2019), perhaps the strangest are where the eggs are incubated either in the mouth or the stomach of a parent.

Since mouth-brooding is known from nine families of fish, most commonly in the cichlids, it is perhaps not so surprising that it has also evolved in frogs. However, biologists were astonished in the 1970s to learn of a genus of Australian frogs (only two species) where eggs are incubated in the stomach of the mother.

After fertilisation of the large (about 5mm diameter) eggs, they are swallowed by the mother and pass to her stomach. A prostaglandin E signal in the mucus coating of each egg (and later released from tadpole gills) inhibits secretion of stomach acid and enzymes. Tadpoles develop and metamorphose in the stomach, entirely fuelled by egg yolk, and froglets are born by regurgitation at about six weeks.

The two gastric-brooding frogs Rheobatrachus silus (southern species) and R. vitellinus (northern, known as the Eungella gastric-brooding frog, from the national park where it was found) lived in rainforest creeks in restricted areas of eastern Queensland. Sadly, both species became extinct soon after their discovery. R. silus has not been seen, despite repeated surveys, since the late 1970s, R. vitellinus since the mid-1980s. A well-planned expedition in 2021 checked previous locations in Eungella and found plenty of suitable habitat, and also populations of two endangered endemic frogs, the Eungella tinker frog and the tusked frog, but no R. vitellinus. Habitat destruction and chytrid infection are both thought to have contributed to steep population declines and eventual extinction.

R. silus has not been seen, despite repeated surveys, since the late 1970s

All may not be lost. Australian scientists have access to frozen Rheobatrachus specimens, and have had some limited success so far in attempting to regenerate embryos by cloning methods, where Rheobatrachus DNA is injected into host nuclei. Whether this work will result in viable reproductively-capable adults has yet to be seen.

The conservation position is a little better for the mouth-brooders. Again, only two species are known, Darwin’s frog (Rhinoderma darwinii ) and the closely related R. rufum. These frogs became known to science from specimens collected by Darwin during the voyage of The Beagle in 1834: the first species was described and named by Dumeril in 1841; the second species was described in 1902. Both species are endemic to the temperate rainforests of central and southern Chile, extending a little into neighbouring Argentina, with R. rufum having a more northern distribution than R. darwinii; there is a small area of overlap where both have been found. In Rhinoderma, after fertilisation of eggs, deposited on to land, the male takes them into his vocal sac and incubates them there: in R. darwinii, they develop through metamorphosis and are ‘born’ as froglets; in R. rufum, they emerge as tadpoles and complete development in water.

The original forested areas inhabited by the two Rhinoderma species have been much altered by anthropogenic development: urbanisation, commercial forestry and agriculture. Sadly, R. rufum has not been seen since 1981 despite repeated surveys, and is now classed by IUCN as ‘possibly extinct’. The effort to save this unusual kind of frog now concentrates on R. darwinii.

R. rufum has not been seen since 1981 despite repeated surveys, and is now classed by IUCN as ‘possibly extinct’

After several decades where steep declines in R. darwinii populations were recorded, the Chilean section of the IUCN’s Amphibian Specialist Group convened a meeting in 2017 aimed at developing a comprehensive conservation strategy. Thirty experts, including the UK’s Andrew Cunningham, contributed to the resulting plan, launched in 2018 ( Azat et al. 2021). The species was estimated to occupy 65 extant populations in Chile and 10 in Argentina with an overall area of occupancy of only 224 km2. Threats to its continued existence include a) habitat loss: little of its original habitat (coastal deciduous forest ) remains, having been replaced by exotic pine and eucalyptus plantations or agriculture; b) chytrid infection has been in Chile since the 1970s, according to analysis of museum specimens, and surveys have shown it can be lethal for R. darwinii ; c) modelling of climate change suggests that current areas where the species persists will be unsuitable in the future; while new areas may be suitable, the frog’s low dispersal ability will limit any benefit from that. Chile and Argentina combined have 30 protected areas where R. darwinii occurs; there are also three private parks where in situ conservation projects are in progress, and also two separate ex situ projects in Chile: Bourke (2010) reported on the establishment of both indoor and outdoor terraria at Concepcion University, with successful breeding occurring; Fenolio (2012) described a separate facility at the National Zoo of Chile, also with breeding success. The agreed conservation strategy includes 39 prioritised actions and aims to fill information gaps, reduce the main threats and achieve legal and financial support  by 2028.

However, a survey in 2024 of one of the largest R. darwinii populations found a 90% decline compared with the previous year. In response, a London Zoo EDGE (Evolutionarily Distinct Globally Endangered Species) team visited the main island of the Chile Archipelago and over five weeks collected 55 frogs from Parque Tantauco, a 1180 km2 private nature reserve. They kept the frogs in quarantine long enough to detect any chytrid infections (52 were chytrid-free), then, using specially-designed climate-controlled boxes, transported the frogs 7000 miles (by boat, road and plane) to London, arriving on 9th December 2024. Eleven of the males were brooding embryos during the trip, and 33 froglets were born soon after arrival in London. The plan is to breed this population in disease-free conditions as a way of rescuing a species whose future in its normal environment looks extremely gloomy. As with all ex situ conservation projects, the question is: what then?  Will it be possible to re-introduce specimens to the wild and then regenerate native populations, and if not, what is the long-term fate of the captive population?

References

Azat et al. (2021). A flagship for austral temperate forest conservation: an action plan for Darwin’s fogs brings key stakeholders together. Oryx 55, 356-363.

Bourke (2010). Darwin’s frog captive rearing facility in Chile. Froglog 94, 2-6.

Fenolio (2012). The Darwin’s frog conservation initiative. Amphibian Ark Newsletter 18, 22-23.

Reproduction without sex: some mother reptiles can do it on their own.

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Roger Downie, Froglife and University of Glasgow

Recently, media outlets summarised the findings of a paper published in Biology Letters (Booth et al., 2023) that reported the first known case of ‘virgin birth’ in a crocodile. Here, I describe the case and put it into the context of other known examples of reproduction without sex in reptiles.

An 18 year old American crocodile (Crocodylus acutus) had been in captivity, in isolation from other crocodiles, in a Costa Rican reptile park since the age of two. To the keepers’ surprise, it produced a clutch of 18 eggs. These were incubated artificially for 3 months, but failed to hatch. They were then opened up: only one contained recognizable contents, a well-developed baby crocodile, sadly dead. Its gonads showed it to be a female, and genetic analysis demonstrated that it was the offspring only of its mother, with no male input.

The technical term for virgin birth, where a female produces offspring without input from a male is ‘parthenogenesis’, and it can be of two general kinds: a) obligate, where it is the only kind of reproduction occurring in that species; b) facultative, where both sexual and non-sexual reproduction can occur.

How common is parthenogenesis? Obligate parthenogernesis is known in about 80 unisexual species of vertebrates, most of them lizards (Neaves and Baumann, 2011). The best-known examples are the whiptail lizards of Mexico and the southern states of USA, with 12 out of over 40 described species of Aspidoscelis (formerly Cnemidophorus) unisexual; and the rock lizards of the Caucasus mountains, with 7 out of 30 Darevskia species unisexual (Spangenburg et al., 2020). The unisexual lizards are nearly all hybrid clones resulting from matings between related species. The unisexual species have either three (triploid) or four (tetraploid) sets of chromosomes which can be identified in terms of the parental species contributing to the cross. It is believed that the unisexual species overcome any disadvantage from the lack of sexual reproduction through the hybrid vigour derived for their extra sets of genes. In Aspidoscelis, Crews et al. (1986) found that the females of unisexual species show female-female courtship behaviour and pseudo-copulation which enhances ovulation.

Obligate parthenogenesis is relatively easy to detect from the complete lack of males in populations of these unisexual species. Facultative parthenogenesis (FP) is more problematic, partly because many species have long-lived sperm which females can retain in their reproductive tracts after mating. The best evidence therefore comes from captive females kept in the absence of males, as in the crocodile case above. It is the first reported example of FP in any crocodilian, and also the first case from a species with temperature-dependent sex determination. In crocodiles, eggs incubated at low and high temperatures develop as females, with males appearing from incubation at intermediate temperatures: the Costa Rican crocodile’s eggs were incubated at 29-300 C, in the female-determining range. FP has been reported from two captive Komodo dragons (Varanus komodoensis) in Chester and London zoos respectively (Watts et al., 2006). The authors note that Komodo dragons kept for the captive breeding of this endangered species are usually housed as single females, with males brought in when the females are in good condition. Their discovery of eggs laid by unmated females (25 eggs, 8 viable in one case; 4 eggs, one viable in the other) may get in the way of the breeding programme. Komodo dragons have ZZ/ZW sex chromosomes, rather than the XX/XY pattern we are more familiar with in mammals. In ZZ/ZW, males are ZZ and females ZW (in XX/XY, males are XY and females XX). Parthenogenetic development from a female Komodo dragon produces ZZ or WW individuals, with ZZ male, and WW probably non-viable. Booth et al. (2012) published the first report of FP in wild snakes: they collected 22 pregnant copperheads (Agkistrodon contortrix) and 37 cottonmouths (A. piscivorus) and examined the litters for signs of FP (few if any male offspring; high proportion developmental failures). Two litters were selected for molecular analysis, to compare the offspring and maternal genomes, and both indicated that they were the products of only the maternal genomes. The authors note that in both cases, one theory for the occurrence of FP did not fit the facts; it has been suggested that females may use FP to overcome a shortage of males in a population, but in neither case was this true. Although unisexuality with obligate parthenogenesis has some advantages over sexual reproduction, it is not clear why FP should occur as an occasional variant on normal sexual reproduction. The finding of FP in a crocodile means that the only reptile taxon where parthenogenesis is unknown is the Chelonia.

The means by which parthenogenesis works as a reproductive mode can be complex. For example, the entry of the sperm into the ripe egg is usually the trigger for development to begin, so how is development triggered in the absence of fertilization? And what happens to the normal chromosomal reduction divisions of meiosis, when no sperm arrives to restore the diploid number? The references to this article will lead you to some of the answers.

You might also ask: does parthenogenesis also occur in amphibians? So far, it has not been reported, but there are unisexual species of amphibians, though they do not reproduce by parthenogenesis: a story for another day.

References

Booth et al. (2012). Facultative parthenogenesis discovered in wild vertebrates. Biology Letters 8, 983-5.

Booth et al. (2023). Discovery of facultative parthenogenesis in a new world crocodile. Biology Letters 19, 20230129.

Crews et al. (1986). Behavioural facilitation of reproduction in sexual and unisexual whiptail lizards. PNAS 83, 9547-9550.

Neaves and Baumann (2011). Unisexual reproduction among vertebrates. Trends in Genetics 27, 81-88.

Spangenburg et al. (2020). Cytogenetic mechanisms of unisexuality in rock lizards. Scientific Reports 10, 8697.

Watts et al. (2006). Parthenogenesis in Komodo dragons. Nature 444, 1021-2.

The astonishing diversity of reproductive modes in amphibians: a new classification

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Written by Roger Downie, Froglife Trustee and University of Glasgow

In the UK, we are accustomed to amphibians breeding in the spring and depositing their eggs in freshwater bodies, usually ponds rather than streams or lakes. Frogs deposit their eggs as a clump of jelly; toads as strings; and newts wrap theirs individually in folded leaves. The embryos hatch as larvae and feed in the water until they are ready to metamorphose into juvenile versions of the adult form. The adults spend no time with their eggs after deposition. So far, so familiar. But, when we look beyond our UK species, we find a wide diversity of reproductive modes. How many, and what are they like?

The term ‘reproductive mode’ (RM) was coined by Breder and Rosen (1966) to help them make sense of reproductive diversity in fish. Later, Salthe and Duellman (1973), in the context of amphibians, defined RM as a set of characters including oviposition site, ovum and clutch characteristics, rate and duration of development, stage and size of hatchlings, and type of parental care, if any. Without using the term RM, Boulenger (1886) had identified 10 amphibian modes. A hundred years later, Duellman and Trueb’s (1986) textbook recognised 29 RMs in anurans, seven in urodeles and two in caecilians. Haddad and Prado (2005) extended this to 39 modes for all amphibians, and there have been a few additions since. However, Nunes-de-Almeida et al. (2021) have now published a new classification, identifying 74 RMs in amphibians, almost a doubling of the 2005 list. How and why?

Their method is to divide the reproductive process into a set of eleven characters where each species can be assigned to one of two (occasionally more) states. The characters are:

  1. Reproduction type: oviparity (egg-laying) or viviparity (eggs not laid: the female gives birth to larvae or juveniles). Viviparity is common in caecilians, but also occurs in a few frogs and salamanders.
  2. Oviposition macrohabitat: eggs are deposited into the environment or they develop in or on the body of either the female or the male parent.
  3. Spawning type: the distinction here is between cases where eggs are immersed in froth, or not. Froth is made from oviduct secretions in two ways: either a foam is generated by beating movements of the adults’ limbs; or bubbles are made by the female’s jumping movements.
  4. Oviposition substrate: either in water, or not in water: on the ground, or in vegetation, or attached to a parent.
  5. Medium surrounding the eggs: the main distinction here is between two kinds of aquatic habitat: lentic (still water, like a pond) or lotic (flowing waters, such as streams). The medium can also be air, as in eggs deposited on the ground, or attached to a parent’s body.
  6. Nest construction: a constructed nest is defined as a place to deposit eggs which the parents have made by digging, or cleaning, or building in some way. ‘Froth’ nests are excluded from this category (I’m not sure this exclusion is fully justified). Constructed nests can be burrows, or depressions, or cleared areas on the forest floor, or leaves folded around the eggs.
  7. Oviposition microhabitat: here, Nunes-de Almeida and colleagues find 15 variables: eggs on the surface of water, at the bottom of a pool, on the ground, on a leaf, on a rock, in a bromeliad tank etc.

The remaining characters distinguish different patterns of development:

  1. Embryonic development: can be indirect, with a larval stage, or direct – lacking a distinct larval form, and progressing directly from embryo to juvenile.
  2. Embryonic nutrition: all amphibians have yolky eggs, and the yolk provides the nutrients needed for embryonic development, but in some cases the mother provides additional nutrients. Where all nutrients derive from the yolk, development is termed lecithotrophic; where the mother provides extra, it is matrotrophic.
  3. Larval and newborn nutrition: when embryos hatch and become free-living, we consider them as larvae. Generally, this marks the stage when they begin to forage for food, although they still have some of the egg-yolk left. However, some species do not feed as larvae, but obtain their nutrition from their large remaining yolk reserves: these are termed endotrophic. Most larvae are exotrophic, obtaining most of their nutrition from external food sources. In a few cases, parents provide this nutrition. For example, so-called trophic eggs, unfertilised eggs deposited by females to feed their hatched larvae. Another example is the feeding of some caecilian young on their mother’s skin secretions.
  4. Place of larval development: mostly this occurs either in a pool (lentic) or a stream (lotic), but there are also cases of larval development on land, or attached to a parent’s body.
Credit: Julia Page

Overall, the authors reviewed RMs in 2171 species on which they could find adequate information: this is 26 % of all amphibians (8393 species, November 2021). Anurans showed 71 of the 74 RMs; urodeles 16 and caecilians seven. Most species showed a single RM, but some fitted up to four of the modes.

Nunes-de-Almeida and colleagues have made a valiant effort to classify the rich diversity of amphibian RMs, but it is not without some problematic aspects. One omitted feature is fertilisation mode: internal or external. This is a crucial feature in research on reproductive strategies relating to certainty of paternity and male competition. Another aspect largely omitted is parental care behaviour. Parental care can be defined as non-gametic investments in offspring that incur a cost to the parent, but which provide a benefit to the offspring. Parental care in amphibians is discussed in Croaking Science (date to come). The new RM classification  explicitly excludes parental care on the grounds that parental care information is lacking for too many species. However, many kinds of parental care are actually included: for example, the provision of trophic eggs to larvae (character 10 above); while others such as larval transportation by adults are omitted. Another omitted feature which I find surprising is the differences in anuran spawn characteristics: single non-adhesive eggs, eggs in clumps, eggs in strings. It is likely that these differences are evolved characteristics important to reproductive success, so should be included in a classification of RMs. Another omission is the diversity of larval forms: there is huge diversity in tadpole form and behaviour, related to the habitats they live in: this may go beyond the usual definition of an RM, but is an important aspect of reproductive success. There are also occasional inconsistencies: phyllomedusine tree frogs wrap their egg clutches in leaves, and this is classed as a constructed nest (character 6 above); newts wrap their eggs individually in leaves, but this behaviour is not acknowledged as a kind of nest construction.

One excellent point made by the authors is about plasticity: i.e. individuals within a species may vary their RM, depending on circumstances. One example I’ve observed is the giant tree frog Boana boans. These frogs generally construct nests, as basins in gravel or sand (character 6 above), just beyond the edge of streams. However, where there is no suitable ‘beach’, the eggs are deposited at the water surface amongst emergent vegetation.

I’m sure that this new RM classification will stimulate discussion and research, and that later versions will include more species and modes. The authors hope that their work will stimulate the development of RM classifications for other taxa: how about reptiles?

References

Breder and Rosen (1966). Modes of Reproduction in Fishes. Natural History Press, New York.

Duellman and Trueb (1986). Biology of Amphibians. Johns Hopkins University Press, Maryland.

Haddad and Prado (2005). Reproductive modes in frogs and their unexpected diversity in the Atlantic forest of Brazil. Bioscience 55, 207-217.

Nunes-de-Almeida et al. (2021). A revised classification of the amphibian reproductive modes. Salamandra 57, 413-427.

Salthe and Duellman (1973). Quantitative constraints associated with reproductive modes in anurans. Pp 229-249 in: Vial (ed.) Evolutionary biology of the anurans. University of Missouri Press, Columbia.

Croaking Science: Cave Salamanders

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Cave salamanders

Cave salamanders belonging to the amphibian genera Hydromantes and Eurycea occur in southern Europe and widely across the United States. There are at least 11 species of salamander which spend their entire lives in cave environments and many more which complete part of their life-cycle in underground environments e.g. fire salamander (Salamandra salamandra). Many salamanders which live in cave environments exhibit two key characteristics: lack of lungs and direct development (i.e. loss of aquatic larval stage). Having no lungs forces these salamanders to rely on absorbing oxygen across their skin and use bucco-pharangeal breathing, which is obtaining oxygen through the moist lining of the mouth. This type of breathing can only occur effectively in specific conditions of high moisture and cool temperature, which are often found in cave environments. Due to their underground existence, the ecology and behaviour of cave salamanders is poorly understood and recent research is only just beginning to understand the biology of these species.

Figure 1. The Ambrosi Salamander (Hydromantes ambrosii) occurs throughout the French and Italian Alps to altitudes of 2,432 m. It does not rely on water for reproduction and often frequents caves and other underground systems to avoid desiccation.
[Photo credit: Benny Trapp, https://commons.wikimedia.org/wiki/File:BennyTrapp_Ambrosis_H%C3%B6hlensalamander_Speleomantes_ambrosii_Italien.jpg]

Previous research on cave salamanders of the genus Hydromantes in southern Italy has shown that these salamanders may reproduce at any time of year, being unconstrained by seasonal environmental changes that affect most non-cave dwelling species. It appears that in most species the female lays a small number of large eggs and protects them until hatching, guarding them from predators and preventing fungal infections (Figure 2). However, the detail of reproductive behaviours such as the number of breeding attempts and nest site selection remain unknown. Recently, Lunghi et al. (2018) carried out a four year study into cave salamanders within the genus Hydromantes at 150 sites in southern Italy. Lunghi et al. (2018) found that in several Hydromantes species the females may retain eggs for many months before releasing them, confirming that the large eggs require a long time to mature in the female before laying. This is followed by a period of at least 6 months of attending eggs and then parental care post-hatching. Therefore, these species require at least two years to complete one reproductive cycle. Given a life-span of only 11 years and maturity at 3-4 years of age, this restricts the number of available reproductive attempts by each female and limits population growth, which is of concern in threatened or vulnerable species. Lunghi et al. (2018) also found that nest site selection was extremely important and that each cave only had a limited number of suitable sites for nesting. Females would occupy only a small number of sites which would vary depending on the year and environmental conditions. This is supported by Lunghi et al. (2014) who found that the wrong choice of nest site can cause breeding failure. Successful breeding seems to occur in nest sites that have high humidity and low temperature. On a yearly basis, different females may use the same nesting site since suitable nests are highly limited. This further suggests that in some species population growth may be limited by suitable areas to breed.

Figure 2. The Italian cave salamander (Hydromantes italicus) lays between 6 and 14 eggs, which take 10 months to hatch into juveniles which resemble the adults (direct development). [Photo credit: Benny Trapp, https://commons.wikimedia.org/wiki/File:BennyTrapp_Speleomantes_italicus.jpg]

Living in cave environments can be challenging for salamanders as often prey densities are low. Relying on freshwater invertebrates may not always provide enough nutrition for species inhabiting these environments. Recent research by Fenolio et al. (2006) has shown that the Ozark Blind Cave Salamander (Eurycea spelaea) exploits another, less obvious food source. During a study to investigate the community ecology of bat caves in Oklahoma, USA, they found to their surprise that the larval salamanders were consuming bat guano. Initially the researchers thought that the ingestion of guano was incidental since consumption of non-food items in amphibians usually only occurs by accident whilst consuming their normal invertebrate prey. Fenolio et al. (2006) investigated this further and found that the salamanders were actually deliberately ingesting the bat guano as a food source. Nutritional analyses of bat guano revealed that it contained nutrients roughly equivalent to those that would be found in a potential prey item in this ecosystem, amphipods. The researchers also found that numbers of E. spelaea increased significantly in the main parts of the cave system where grey bats deposit fresh guano. This study contradicts the general understanding that salamanders are strictly carnivorous and shows that this adaptation enhances the survival of this species in this harsh environment.

Figure 3. The Ozark Blind Cave Salamander (Eurycea spelaea) of Missouri and southern Kansas, USA lives in caves and feeds on bat guano for nutrition.
[Photo credit: Peter Paplanus, https://commons.wikimedia.org/wiki/File:Eurycea_spelaea,_Izard_County,_Arkansas,_by_Peter_Paplanus.jpg]

Cave salamanders may also exhibit several morphological adaptations to living in dark and humid underground environments. The Ozark Blind Cave Salamander is also unique in that it starts life as a fully sighted larva but then metamorphoses underground into a terrestrial adult that loses its pigment and becomes blind, with the eyelids eventually fusing (AmphibiaWeb, 2016). There are several hypotheses to explain the reason for this morphological change. However, the most favoured is the energy economy hypothesis which states that the cost of developing and maintaining eyes is substantial and a large portion of the brain is needed for visual processing with high oxygen consumption rates. Energy savings from the loss of eyes could reduce the amount of time needed for foraging and allow energy to be re-invested into other physiological processes, including reproduction (AmphibiaWeb, 2016). In addition, eye loss may be linked to the process of skull development which is a pattern also observed in blind cavefish.

Cave salamanders are a unique group of amphibians with highly evolved ecology and life history characteristics. Unfortunately, an increasing number of cave salamanders are now threatened and classified as Vulnerable or Endangered on the IUCN Red List. Increasing our understanding of the ecology and biology of these declining species is critical in developing effective conservation initiatives, especially in cave environments which are prone to human-induced and climatic change.

References

AmphibiaWeb (2016) Eurycea spelaea: Grotto Salamander <http://amphibiaweb.org/species/4220> University of California, Berkeley, CA, USA. Accessed 27 September, 2019.

Fenolio, D.B., Graening, G.O., Collier, B.A. & Stout, J.F. (2006) Coprophagy in a cave-adapted salamander; the importance of bat guano examined through nutritional and stable isotope analyses. Proceedings of the Royal Society, London B, 273: 439–443.

Lunghi, E., Manenti, R., Manca, S., Mulargia, M., Pennati, R. & Ficetola, G.F. (2014). Nesting of cave salamanders (Hydromantes flavus and H. italicus) in natural environments. Salamandra, 50 (2): 105-109.

Lunghi, E., Corti, C., Manenti, R., Barzaghi, B., Buschettu, S., Canedoli, C., Cogoni, R., De Falco, G., Fais, F., Manca, A., Mirimin, V., Mulargia, M., Mulas, C., Muraro, M., Murgia, R., Veith, M. & Ficetola, G.F. (2018) Comparative reproductive biology of European cave salamanders (genus Hydromantes): nesting selection and multiple annual breeding. Salamandra, 54 (2): 101-108.

Croaking Science: Unisexuality- an alternative reproductive strategy

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Approximately 90 vertebrate species are known to exist as unisexual populations, that is, consisting of reproducing females (Lampert & Schartl, 2010). All of these are restricted to fish, amphibian and reptile species (Figure 1). An all-female reproducing population has advantages since every individual can carry young so the population has the potential to grow at a faster rate than bisexual populations, where the male only donates sperm and does not produce offspring. Since this has a distinct advantage it is perhaps unusual that unisexual populations are so rare. However, the main cost of being unisexual is the loss of genetic recombination which occurs in male and female bisexual populations. This is crucial in providing genetic stability, preventing accumulation of mutations and providing opportunities for adapting to a changing environment. Therefore, unisexual species do not occur widely amongst vertebrates.

Figure 1. The Caucasian rock lizard is one of a few lizards which are unisexual
[Photo credit: Alastair Rae, https://commons.wikimedia.org/wiki/File:Caucasian_Rock_Lizard_(34598190795).jpg]

Unisexual reproduction is often referred to as parthenogenesis, which is “reproduction in the absence of fertilization of the egg” (Lampert & Schartl, 2010). However, there are different forms of parthenogenesis which do allow some genetic exchange. True parthenogenesis occurs in females in the complete absence of males. This is incredibly rare and is known only from a handful of vertebrate species. However, one other type of parthenogenesis, known as gynogenesis, occurs when male sperm is used to trigger development of the female’s embryo. In this circumstance, the female does not incorporate any of the sperm’s genetic material and she uses all of her own so the sperm’s genome is redundant. However, in some situations, known as kleptogenesis, the female may incorporate some of all of the sperm’s genome thus resulting in offspring that have, 2, 3 or 4 times the amount of genetic material (diploid, triploid or tetraploid respectively) (Bogart et al., 2007). Since they ‘steal’ gametes of sexual species for their own reproduction, they are considered to be sexual parasites of their (usually parental) host species (Mikulíček et al., 2014). A third type of parthenogenesis is hybridogenesis where the eggs of a unisexual female are fertilised by the sperm of a closely related bisexual male. However, this genetic material is only incorporated into the new genome for one generation and is excluded when this female produces her own eggs (Lambert et al., 2010).  Therefore, only maternal genes are passed onto subsequent generations. All these forms of parthenogenesis result in the inclusion of ‘fresh’ genetic material into the female thus improving the fitness of populations and increasing their robustness to resist diseases.

Figure 2. Four species of Ambystoma salamander which may be included in the nucleus of unisexual individuals. [Photo credit: Bogart et al., 2007.]

Two main groups of amphibians are known to exhibit unisexuality. Unisexual mole salamanders (genus Ambystoma), occur in North America and generally have between two and five times the normal complement of genetic material compared to bisexual Ambystomid salamanders (Figure 2). These Ambystomid unisexual individuals are the oldest known group of unisexual vertebrates, having occurred for over 5 million years (Gibbs & Denton, 2016). The DNA in the nucleus of unisexual individuals is usually composed of genetic material from several species including: the blue spotted salamander (Ambystoma laterale), Jefferson salamander (A. jeffersonianum), small-mouthed salamander (A. texanum) and tiger salamander (A. tigrinum). This leads to a large range of possible genetic combinations with genetic material from the blue spotted salamander being present in most unisexual individuals. Female unisexual salamanders require sperm from a bisexual salamander species which are living in the same area to initiate reproduction. However, they can then either use the sperm solely to activate egg development (i.e. gynogenesis) or incorporate the sperm genome into the resulting offspring (Bogart et al., 2007). If genetic material from the sperm is used during reproduction, either the DNA is retained, resulting in the offspring with additional genetic material, or it is replaced. In a recent study, Gibbs & Denton (2016) studied the genetic exchange in unisexual populations of Ambystomid salamanders to try and explain how unisexual populations have persisted in the environment for 5 million years. They found that unisexual individuals gain enough genetic material through the occasional process of obtaining DNA from males to allow populations to remain robust and able to withstand environmental change. Therefore these individuals gain all the advantages of being unisexual, with also some of the advantages of sexual reproduction i.e. genetic mixing.

Figure 3. Two edible frogs (Pelophylax esculentus). [Photo credit: Awewewe,]

One of the well-known breeding systems involving hybridogenesis is that of the water frog (Pelophylax esculentus) complex (Ranidae), widely distributed in Europe, which has considerable variation in types of hybridogenesis (Figure 3). The Pelophylax esculentus complex consists of two parental species, the marsh frog (P. ridibundus) and pool frog (P. lessonae), and their hybridogenetic hybrid the edible frog (P. esculentus). Edible frogs usually contain genetic material from both parental species. In most of its range, the edible frog reproduces hybridogenetically with the pool frog. However, during reproduction the pool frog genome is lost in the eggs and sperm. Therefore, pool frog DNA is not passed down to subsequent generations and the edible frog is considered a sexual parasite (Christiansen et al., 2005). However, many variations of this mating system occur across the species’ range. In most cases, female edible frogs will use the genetic material from the male, but it is not fully incorporating it into its genome. Due to the unusual genetic combining during hybridogenesis the presence of edible frogs can result in loss of the parental pool frog and marsh frog genotypes and is a potential conservation problem when it occurs in these populations.

Overall, unisexuality in amphibians represents a unique form of reproduction that has remained evolutionarily stable for several million years through occasional genetic mixing from the same or closely related species. Ambystomid salamanders and water frogs have proved highly successful at unisexual reproduction which is otherwise a rarely used breeding strategy amongst vertebrates.

References

Bogart, J.P., Bi, K., Fu, J., Noble, D.W.W. & Niedzwieckim, J. (2007) Unisexual salamanders (genus Ambystoma) present a new reproductive mode for eukaryotes. Genome, 50: 119–136.

Christiansen, D.G., Fog, K., Pedersen, Bo V. & Boomsma, J.J. (2005) Reproduction and hybrid load in all-hybrid populations of Rana esculenta water frogs in Denmark. Evolution, 59 (6): 1348–1361.

Gibbs, H.L., & Denton, R.D. (2016) Cryptic sex? Estimates of genome exchange in unisexual mole salamanders (Ambystoma sp.). Molecular Ecology, 25: 2805–2815.

Lampert, K.P. & Schartl, M. (2010) A little bit is better than nothing: the incomplete parthenogenesis of salamanders, frogs and fish. BMC Biology, 8: 78-80.

Mikulíček, P., Kautman, M., Kautman, J. & Pruvost, N.B.M. (2014) Mode of hybridogenesis and habitat preferences influence population composition of water frogs (Pelophylax esculentus complex, Anura: Ranidae) in a region of sympatric occurrence (western Slovakia). Journal of Zoological Systematics & Evolution Research. doi: 10.1111/jzs.12083.