Other Parthenogenic Species
Does Virgin Birth Exist?
A group of microscopic aquatic animals has evolved into many separate species over the past 40 million years without sexual reproduction. The research focused on bdelloid rotifers, which live in ponds, rivers, and occasionally wet habitats like soils, mosses, and lichens. Biologists Ridego Fontaneto from the University of Milan and Timothy Barraclough from Imperial College London found evidence of distinct species of the bdelloid rotifers by comparing DNA sequencing and jaw measurements of animals living across the U.K., Italy, and other parts of the world. Asexual animals can evolve and mutate over time, but usually do not diversify or last long. However, records of the bdelloid rotifers show they have been around for more than 40 million years and they have evolved into hundreds of species individually adapted the their environments allowing some to now live in hot springs or the Antarctic waters. (CBC News, 2007).
Mammals were apparently the only group of animals among all known jawed vertebrates that could not undergo parthenogenesis. Yet an experiment was described by Kono et al. which resulted in these parthenogenetic mice. They examined what made these mice so resistant to parthenogenesis and realized that it was because of the expression of epigenetic factors that regulated development in early mammalian embryos. Through this experiment, 457 out of 598 treated oocytes that had begun their cleavage stages were artificially activated. More than 80% of the activated eggs produced blastocysts and these were transferred to 26 recipients. This then resulted in 10 live and nine dead offspring, of which one developed to an adult. This adult had now reached 14 months of age and had delivered her own offspring. However, parthenogenesis has its own problems with a declining genetic diversity in the species of interest which in turn restricts the ability of the parthenogenotes to adapt to a new environment. (Edwards, 2007).
Komodo dragons, the largest of the lizards, are under threat as wild populations become smaller. Genetic fingerprinting was used to identify parthenogenetic offspring produced by two female Komodo dragons that had been kept at separate institutions and isolated from males. One of these females then produced additional offspring sexually. This indicates that Komodo dragons can switch between asexual and sexual reproduction. There are only two sexually mature female Komodo dragons in Europe. One of the dragons had never been kept with a male but has produced a clutch of 25 eggs, of which 11 seemed to be viable. Three of the eggs collapsed early during incubation but provided embryonic material able for genetic use. The remaining eight eggs are developing normally. It was found that all offspring produced in the absence of males were parthenogens, but they were not identical clones. (Watts et al, 2006).
Parthenogenesis among reptiles is rare. Only a few species have the ability to reproduce asexually. A female python from the Artis Zoo in Amsterdam produced eggs in five consecutive years that contained embryos while she was isolated from a male. Four microsatellite loci developed for this species combined with three loci developed previously for different snake species reveled too little variation to discriminate between sperm retention and parthenogenesis. With AFLP analysis, they were able to confirm whether it was sexual or asexual reproduction.
In snakes, females are the heterogametic sex (ZW). Homozygous offspring will be either WW or ZZ. Since the WW combination is not available, all observed offspring are ZZ and thus are males. The Artis Zoo in Amsterdam contains three adult female Burmese pythons but no males. One female was born in 1993 and has been separated from males since 1995. She has produced a single clutch every spring starting in 1997 up to 2002. Healthy embryos have been observed in 25-30% of the eggs every year. No material of the 1997-2000 clutches had been saved for later use in a genetic study. Seven embryos were isolated from eggs of the 2001 clutch after a 24 day incubation period. The results of this test showed that six out of seven microsatellite loci were monomorphic and all individuals were homozygous for the same allele. All seven young had identical fingerprints, and all the markers of the offspring were also found in their mother. So the young are genetically identical to their mother and do not show markers that suggest the involvement of a father. These observations then suggest that this female python is parthenogenetic. (Groot et al, 2003).
The Cape bee is unique among honeybees in that workers can lay eggs that instead of developing into males develop into females via thelytokous parthenogenesis. This ability allows workers to compete directly with the queen over the production of new queens. Genetic analyses using microsatellites showed that 23 out of 39 new queens produced by seven colonies were offspring of workers and not the resident queen. Of these, eight were laid by resident workers, but the majority was offspring of parasitic workers from other colonies. The parasites were derived from several clonal lineages that entered the colonies and successfully targeted queen cells for parasitism. The eggs are produced by meiosis, but then the polar body nucleus fuses with the egg nucleus restoring diploidy. Of the daughter queens laid by the resident queen, three were produced asexually, suggesting that queens can choose to produce daughter queens clonally and thus have the potential for genetic immortality. (Jordan et al, 2007).
In a study, hybrid and non-hybrid parthenogenetic and sexual snails were compared for the following characteristics: female size-fecundity curves, offspring size, survivorship, and growth. Compared to nearby sexual populations, triploid hybrid parthenogens from the Florida Gulf coast have similar fecundity and offspring size, five- times higher survivorship, and 60% higher growth. Relative to nearby sexual populations, non-hybrid parthenogenetic snails from the Atlantic coast have significantly higher fecundity, smaller offspring size, similar survivorship, and slightly lower growth. Given the considerable fitness advantages of parthenogens, especially in the hybrid parthenogens, it is obvious why parthenogenesis is used. (Johnson, 2005).
In mated or inseminated turkeys, 5-15% of eggs set for incubation show rudimentary development. Most of these embryos die during the first 24 to 48 hours of incubation and contain only unorganized sheets of tissue. This is termed as positive development (PD). Turkey eggs also show incidence of parthenogenesis and the resulting progeny is always thought to be male. Parthenogenetic embryos were obtained from eggs laid by virgin Beltsville Small White (BSW) hens, and the PD embryos were obtained from eggs of inseminated Nicholas and British United Turkeys of America (BUTA) hens. DNA was extracted from blastoderms of parthenogenetic and PD embryos. Turkey W-chromosome specific DNA probe and primers were used to detect females in all samples by Southern blot and polymerase chain reaction (PCR). No female was detected among the 35 parthenogens examined, whereas there were 3 females among the 11 PD embryos. The presence of both males and females among PD embryos suggests that they are products of fertilization, and that at least these 3 female embryos, if not all the 11 PD embryos, are not of parthenogenetic origin. It is concluded, therefore, that PD embryos result from errors in fertilization or from early embryonic mortality following successful fertilization and that they are unlikely to be of parthenogenetic origin. (Cassar et al, 1998).