Sex and the Single Elegens

 

Sex reproduction has been with us for about 1.5 billion years in one form or another. It involves a complex lifecycle in which a gamete (sperm or egg cell) with a single set of DNA chromosomes combines with another gamete to produce a zygote (fertilized egg) that develops into an organism composed of cells with two sets of chromosomes (diploid).

In humans we have two sex-linked chromosomes, one (X) associated with both males and females and the other (Y) associated only with male male. When the cells that become gametes divide, each gamate (normally) has either an (X) or a (Y) chromosome such that when the zygote is form it it has either a pair of XX chromosomes that result in a female or an XY pair that results in a male. 

C. elegens is normally a hermaphrodite with all cells having a pair of  XX chromosomes that has a reproduction system produces both male and female gametes, each with an X chromosome which results in a fertilized egg with a pair of XX chromosomes such that only hermaphrodites zygotes are produced But in about 0.1% of the cases, C. elegens have a single X chromosome (XO) such that one gamete is produced with an X chromosome and the other with a missing O chromosome. When the second X chromosome is missing, C. elegens zygote develops into a physically different specimen with a reproductive system having testis to produce sperm but no ovary to produce eggs.

Being unable to reproduced on their own, the smaller males mate with females as shown in the photograph above. But since half the sperm produced by the male has the X chromosome and half has the O chromosome and all the eggs produced by the hermaphrodite have an X chromosome equal numbers of XX zygotes and XO zygotes result when the eggs produced only by the hermaphrodite are fertilized by sperm from the male. On the other hand, all the sperm produced by the hermaphrodite have only X chromosomes, all the eggs fertilized by sperm from hermaphrodite. But in the end, the hermaphrodite:males ratio of mature C.elegens averages out to about 999:1.

Humans, of course, have X chromosomes by also Y chromosomes rather than O chromosomes. Those with pairs of XX chromosomes are females and those with XY pairs of chromosomes are males. All female gametes  are eggs with only an X chromosome while all males gametes are sperm, half with an X chromosome and half with a Y chromosomes. Thus half the fertilized zygotes are female with a pair of XX chromosomes and half are male with a XY pair of chromosomes. And so it is that, unlike C. elegens, our male:female ratio of mature fetuses is 1:1.

Interestingly, humans can also develop as hermaphrodites containing both ovarian and testicular tissue. Case of hermaphrodites fathering children and of hermaphrodites bearing children are know but very rare. It is also possible for a hermaphrodite to self fertilize and bear a child but no cases are known. 

And so it is that in C elegens as in humans, sex is sex.