The animal kingdom has evolved a remarkable number of ways to have sex. Males and females, hermaphrodites, and parthenogenic females that get by without males were all familiar to me, but this week's PNAS introduced me to a brand new one: androgenesis, in which a species reproduces using only the DNA from sperm. Now, researchers have looked into the genetics of some androgenetic clams, and found that this method of reproduction has turned them into a bit of a species factory, and kept them from building up too many harmful mutations in the process.
Many of the animal species that reproduce without a mate are hermaphrodites, in which a single individual can produce both eggs and sperm. This process lets them produce offspring even when mates aren't available, and helps them get rid of harmful mutations (and collect helpful ones) through recombination. But some species have gone female-only, with their eggs going on to develop into embryos without any significant contribution from male sperm (the authors cite the bdelloid rotifers, where no males have been observed in the fossil record for 35 million years). Most of these species have evolved ways of picking up DNA from related individuals, allowing some degree of recombination that rescues them from accumulating harmful mutations.
It's easy to see how an egg could develop without the contribution of sperm, since it's full of proteins and nutrients, but having a male-only species seems like a bit of a challenge, since sperm carry little more than DNA and the equipment necessary to move it around. But, in fact, the androgenetic clams of the genus Corbicula do use eggs; they just don't use the eggs' DNA. The species in the genus that reproduce sexually are hermaphrodites, producing both eggs and sperm, and so do the androgenetic species. The difference is that, after fertilization, the eggs of the androgenetic species kick out all the female DNA, and go on to develop solely from the DNA carried by the sperm.