I began writing a series on sex cells when I heard about hammerhead sharks giving birth without the benefit of a male. This type of reproduction is called parthenogenesis, and I tried to explain it, in part, by referring to the Parthenon.
Each row of columns in the Parthenon represents a set of chromosomes. In most cases, an organism needs at least two sets to be able to support a whole organism. The analogy works because you would need two sets of columns to keep the roof on a structure.
Sex cells are interesting because they serve as a chromosome delivery system to the next generation. Normally, these special cells carry only one set of chromosomes. They need to join up with another sex cell to create an embryo.
I have been writing about the exceptions to the rule. In the case of the hammerhead shark, an egg joined up with another cell from the female shark to produce a virgin born shark. Let’s call this true parthenogenesis.
In the case of the Amazon molly, the all female species has eggs that double their own chromosomes to produce a clonal offspring, but they require the stimulation of mating to turn their eggs into embryos. No actual fertilisation takes place, however. This type of reproduction is called gynogenesis.
I want to explain a third type of unusual reproduction called hybridogenesis. It is an apt name for what some fish do: they not only let their sperm hybridise with their eggs, they are hybrids themselves. Some species have two different species’ chromosomes inside their nucleus, and others (the rare triploid species) have three!
Hybridogenesis occurs in several species of the genus Poeciliopsis. Unfortunately, they do not have a common name, and their scientific names are long because they include the names of both of their species of orgin (for example, Poeciliopsis monacha-lucida). Hybridogenetic species actually let the sperm fertilise the egg, but it is a cruel trick, because the male genes are not incorporated into the offspring.
It is an amazing trick, accomplished by the failure of the female cell to recognise the chromosomes of the males. Structures that usually attach to the chromosomes and pull them around the cell fail to recognise chromosomes that came from outside of the egg. Think of it as a bunch of columns without tops and bottoms, unable to stand tall, rolling down the slopes of the Acropolis. The male chromosomes do not stay together as a set, are not passed onto each new cell, and have no genetic role. The fathers of these fish are no better off than the males that mate with the Amazon molly. The baby fish produced have only their mother’s genes.
But hybridogenetic fish are better off in one regard: they have elevated levels of genetic diversity. Because their cells contain sets of chromosomes from two or three species, they start out with a lot of genetic variation. This type of virgin birth has fewer drawbacks from a genetic perspective – the all female species are not at risk of inbreeding depression.
I have summarised (in four articles) three unusual forms of reproduction. It is hard to say which is stranger, the hybrid fish that spit out male genes following fertilisation, the female fish that mate only to remind their eggs to develop, or the sharks that managed to reproduce without any sperm at all.
It is interesting to note that the most extreme version of parthenogenesis was found in a species that usually reproduces sexually in a normal fashion. If they had met a male and got some sperm, their eggs would have known what to do with it, and their babies would have had two parents.
Two parents is the normal situation for most fish and sharks. But unlike humans, they can and do cheat, producing daughters without fathers. These species take single parenthood to a whole new level, proving how diverse this thing called life can be.
Previous articles in the sex cell series
Parthenogenesis in the hammerhead shark