Worm fertilises itself and sheds its genes

New research published in the journal Science indicates that a species of nematode worm has evolved the ability to reproduce without the need for a partner, a phenomenon called “selfing”. 

This is a pretty neat trick, and whenever we encounter such clever adaptations, it tends to feed into a view of evolution that sees the process as one of accumulation – the gaining of complexity and capacities.

But evolution merely entails the adaptation of organisms to changing environments in order to out-reproduce others, and can just as easily lead to loss as gain.

The new research is a case in point. 

Studying the tiny worm Caenorhabditis briggsae, a team of scientists led by biologist Eric Haag from the University of Maryland, US, discovered that evolving self-fertilisation may have cost it a quarter of its genome.

The majority of species comprising “eukaryotic” cells that contain nuclei relies on sexual reproduction, but a C. briggsae is among the few species that have evolved selfing, more formally known as “asexual parthenogenesis”. This has led to most members of them being hermaphrodites with both male and female sex organs.

C. briggsae’s closest evolutionary relative is Caenorhabditis nigoni. The two split roughly a million years ago, with the primary difference between them being that C. nigoni still relies exclusively on sexual reproduction, or “outcrossing”.

The team, led by biologist Eric Haag, compared the genetic sequences of both species and discovered that C. briggsae has 7,000 fewer genes. Curiously, the lost genes appear to be predominantly active in males of C. nigoni, leading the researchers to hypothesise a sex-related function.

This led them to a class of genes that are entirely absent from all known selfing Caenorhabditis species, known as “male secreted short” (mss) genes. They are present in outcrossing species, however, suggesting they may well number among the 7000 lost ones. 

Although the exact function of mss genes remains a mystery, the team’s experimental results strongly implicate them in a form of sexual selection known as sperm competition. This is much like it sounds: when a female has sex with multiple males, the sperm compete to be first to fertilise her eggs. 

In selfing species this is obviously irrelevant.

“The fact that all the selfing species lost the mass genes suggests that these genes, which are very useful for worms having male-female sex, are harmful for worms that are no longer having sex with each other,” says Haag. “What we are seeing is an evolutionary snapshot of how a species fine-tunes its reproduction.”

Importantly for our understanding of evolution, Haag concludes, “our results suggest that genes that are essential for tens of millions of years can suddenly become useless or liabilities, even, when the sex system changes.”