Reproduction takes two… right? This is true for most animal species, with reproduction involving the fusion of a sperm and an egg. However, one fish species has taken a different approach. The Amazon Molly is entirely female. When they are ready to reproduce, members of this species must find a male of another, related species, and mate with him. However, instead of using the sperm from this male to provide half of the genetic information for the future offspring, the female Amazon Molly merely uses the sperm as a signal to trigger embryogenesis, in a process known as gynogenesis.
For most animal species, the fusion of egg and sperm is the defining moment of reproduction, when genetic information from each parent is joined to form the final genetic make-up of the offspring. Each parent produces gametes (sperm / egg) which contain half of the chromosomes of the parent, a state known as haploid. When two gametes fuse to produce a zygote, the two sets of genetic information from each parent join together, producing a diploid egg cell. However, in Poecilia formosa, no genetic information is taken from the male at all. The female is capable of producing her own diploid eggs, and only requires the male to provide a physiological trigger in the form of his sperm.
The males that the Molly is able to trick into mating with her get nothing for their trouble, while she produces offspring who are genetically identical to her. The Amazon Molly might be tricking her mates out of their progeny, but she’s not picky. She will mate with males of different species in different parts of her range.
Sometimes the trick backfires, however. If the mechanisms for clearing away the males sperm after mating fail, then fertilisation of the diploid Molly egg with the haploid sperm may occur, producing triploid (three sets of each chromosome) offspring. These offspring have reduced fitness and are less able to compete with their diploid siblings.
Want to know more?
- Schultz & Kallman (1968) Triploid Hybrids between the All-female Teleost Poecilia formosa and Poecilia sphenops Nature 219, 280 – 282
- Lamatsch et al (2008) Diploid Amazon mollies (Poecilia formosa) show a higher fitness than triploids in clonal competition experiments. Evolutionary Ecology 23, 687 – 697
- Lampert & Schartl (2008) The origin and evolution of a unisexual hybrid: Poecilia formosa. Philos Trans R Soc Lond B Biol Sci. 363, 2901–2909.