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.

To get some insights into the origin of this unusual form of reproduction, the authors of the new paper looked at DNA from a variety of Corbicula species, some of which reproduced as hermaphrodites, others exclusively as males. To get a clearer picture, they looked both at a pair of nuclear genes and at the mitochondrial DNA, which is inherited solely from the egg.

The results, to understate, were a bit confused. The nuclear and mitochondrial genomes produced very nice phylogenetic trees, but they simply weren't the same trees. Attempts to get them to align produced a confused criss-cross of lines rather than a simple, linear relationship. In fact, there was confusion even within the nuclear genome. All of the androgenetic species shared a copy of a closely related set of sequences, suggesting that the species themselves were all closely related, and descendants of a single event that brought about their new form of reproduction. But the copy of the same gene on the other chromosome was often distantly related, and lined up much more closely with the same gene in species that reproduced as hermaphrodites.

The simplest way to sort this out would have been to sequence more genes and hope something easier to interpret became apparent when more information was available. But the researchers chose to put their brains to work instead of the sequencing machines, considering a variety of models for the pattern they saw, and rejecting most of them as inconsistent with either the data or the biology of the clams themselves (one possibility was rejected because the habitats of different clam species don't overlap enough to make it realistic).

The model that was left standing proposes one or possibly two origins for androgenetic reproduction, with that origin having been in the relatively recent past. For the most part, different lineages from these origins have been reproducing asexually and slowly building up minor genetic differences as a result. On rare occasions, however, an androgenetic sperm will run into the egg of a female of a different species (clams generally mate by releasing their sperm into the currents) and undergo sexual reproduction, creating a hybrid species. From there, androgenetic reproduction takes over again, and the hybrid continues to reproduce asexually.

But, in that single, rare event of sexual reproduction, the clam will have picked up a new mitochondrial genome and a set of chromosomes that come from a hermaphrodite species. That swapping of DNA is sufficient to explain the confused pattern of relatedness seen in the phylogenetic trees—each hybrid species is related to at least two parental species. This also suggests that these rare interbreeding events have the potential to keep seeding new species of clams that then can continue multiplying in reproductive isolation from both their parental species.

The other key thing that these hybrids ensure is that there are new sources of genes to help replace any that get damaged through mutation. So, although the origin of these androgenetic species seems to be recent, they seem to have a mechanism in place that may keep them around for the long haul.

PNAS, 2011. DOI: 10.1073/pnas.1106742108 (About DOIs).

Listing image by Wikimedia Commons