Explained: What Are Virgin Births? A Type Of Asexual Reproduction In Sexually Reproducing Species

Facultative parthenogenesis is an unusual mode of asexual reproduction found in sexually reproducing species. It is their ability to generate offspring without genetic contributions of males. 

Taken into captivity in 2002 when she was two years old, an American female crocodile kept in isolation for 16 years laid a clutch of 14 eggs in January 2018. The crocodile exhibited virgin birth, known as facultative parthenogenesis in scientific terms. (Representative Photo)

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An American crocodile (Crocodylus acutus) from Costa Rica has left scientists intrigued because she represents the first recorded case of "virgin birth" in crocodiles. Taken into captivity in 2002 when she was two years old, the crocodile was placed in an enclosure at Parque Reptilandia, a tourist attraction site in Costa Rica, and kept isolated for 16 years. However, on January 17, 2018, the crocodile laid a clutch of 14 eggs. 

She was 18 years old at that time. Of the 14 eggs laid, seven appeared to be fertile. These were artificially incubated for three months. However, the eggs failed to hatch, and were opened to assess their contents. While the contents of six eggs were not discernable, one was found to contain a fully formed non-viable foetus that was determined as female using whole-genome sequencing, a study, published June 7 in the journal Biology Letters, said. 

What is facultative parthenogenesis or virgin birth?

The crocodile exhibited virgin birth, known as facultative parthenogenesis in scientific terms. Over the past two decades, scientists have documented several cases of vertebrate facultative parthenogenesis, in animals such as birds, non-avian reptiles like lizards and snakes, and elasmobranch fishes, which are cartilaginous fish such as sharks, rays and sawfish. 

Facultative parthenogenesis is an unusual mode of asexual reproduction found in sexually reproducing organisms. It is the ability of sexually reproducing species to generate offspring without genetic contributions of males. 

Facultative parthenogenesis is known from a wide range of ordinarily sexually reproducing vertebrates in captivity. According to a study published June 2015 in the journal Current Biology, vertebrate parthenogens are characterised as being of the homogametic sex, such as females in sharks and males in birds, and by having elevated levels of homozygosity, or the presence of identical alleles at a particular gene locus, compared to their mother. This may reduce the viability of the parthenogen. 

According to the study authors, facultative parthenogenesis may have adaptive significance, and if this fact is true, it could be hypothesised that parthenogenesis would be found most often at low population density, an instance in which females risk reproductive failure because finding mates is difficult.

What are the common facultative parthenogenesis characteristics observed in vertebrates?

The study published in Biology Letters said that common facultative parthenogenesis characteristics observed in vertebrates include the fact that the sex of parthenogens (offspring produced as a result of parthenogenesis) is constrained by the species' sex chromosome system. This means that XX females produce only female (XX) parthenogens, and ZW females, a sex chromosomal system observed in some birds, butterflies, fish and crustaceans, where females are the heterogametic sex, produce only male (ZZ) parthenogens. 

Another characteristic commonly observed in vertebrates showing facultative parthenogenesis is that these organisms have the ability to produce both sexual and parthenogenetic offspring within single-reproductive events, the study published in Biology Letters said. Organisms exhibiting virgin birth have the capacity for consecutive parthenogenetic events, and adult parthenogens can perform sexual reproduction as well as secondary parthenogenesis. Wild populations of some vertebrates have borne viable parthenogens.

Temperature-dependent sex determination in crocodiles

Scientists have debated the occurrence of facultative parthenogenesis in taxa such as Chelonia (turtles) and Crocodylia, which includes crocodiles, alligators and gharials. 

The taxa Crocodylia has been of interest to scientists because crocodilians, unlike all previously documented cases of facultative parthenogenesis in vertebrates, lack sex chromosomes, and sex determination is controlled by temperature. This mechanism is called temperature-dependent sex determination.

According to the study, Crocodylus acutus exhibits a female-male-female temperature-dependent sex determination pattern, with temperatures below 30 degrees Celsius and above 33 degrees Celsius producing 100 per cent females, and temperatures around 31.5 degrees Celsius producing a majority of males. 

As part of the study, the researchers incubated the eggs at 29 to 30 degrees Celsius, and therefore, the fully formed yet stillborn foetus was female.

The authors said that temperature-dependent sex determination is an ancestral trait in crocodilians, and has also been observed in turtle, lizard and tuatara lineages. 

What kind of facultative parthenogenesis was observed in the female American crocodile?

The study found that the offspring of the female American crocodile had identical genotypes to the mother at greater than 99.9 per cent of her homozygous loci (location of a specific gene on a chromosome). A homozygous genotype has two identical alleles at a particular gene locus. Of all the mother's homozygous loci, more than 99.9 per cent were identical to those found in the offspring. This demonstrates a lack of paternal alleles.

Heterozygosity, or the presence of two different alleles at a particular gene locus, was 0.349 in the mother, and 0.045 in the offspring. This demonstrates a significant reduction in heterozygosity in the offspring in comparison to the mother. 

The study found that the offspring retained about three per cent of the maternal heterozygosity. This finding suggests that the crocodile may have undergone facultative parthenogenesis through terminal fusion automixis. Automixis is a type of parthenogenesis in which the chromosomes of a haploid gamete divide without nuclear division, resulting in the formation of a diploid nucleus. 

In the case of terminal fusion automixis, the egg nucleus fuses with the second polar body, both of which are haploid. 

The primary oocyte, which is diploid, forms a secondary oocyte and a first polar body, both of which are haploid. The secondary oocyte divides into form an egg and a second polar body.  The fusion of the haploid egg nucleus and the second polar body results in the formation of a diploid organism. This type of facultative parthenogenesis results in reduced levels of heterozygosity in the offspring. Terminal fusion automixis always causes retained heterozygosity in the offspring to be less than 33 per cent of that of the mother. 

Evolutionary significance of facultative parthenogenesis

A reason why facultative parthenogenesis in crocodilians is a matter of interest to scientists is that crocodilians are members of a larger monophyletic (relating to a group of organisms that has descended from a common evolutionary ancestor or ancestral group) lineage called Archosauria, a crown group which at its base splits into two groups. 

These are Pseudosuchia and Avemetatarsalia. Pseudosuchia comprise crocodilians and their extinct relatives, while Avemetatarsalia contains birds and their extinct relatives, including pterosaurs and non-avian dinosaurs. Therefore, the occurrence of facultative parthenogenesis in crocodiles is of evolutionary significance.

The authors noted in the study that the fact that terminal fusion automixis was observed in the crocodile, which has also been found in birds, snakes and lizards, suggests that this is a trait probably possessed by a distant common ancestor of these lineages. 

Facultative parthenogenesis is more common in low-density populations, such as those on the verge of extinction. 

The occurrence of terminal fusion automixis in the crocodile suggests a common evolutionary origin of facultative parthenogenesis across reptiles, crocodilians and birds. The authors concluded that since facultative parthenogenesis has now been documented in the two main branches of extant archosaurs, the discovery offers insights into the possible reproductive capabilities of the extinct archosaurian relatives of crocodilians and birds, especially members of Pterosauria and Dinosauria.