Males as somatic investment in a parthenogenetic nematode

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Science  15 Mar 2019:
Vol. 363, Issue 6432, pp. 1210-1213
DOI: 10.1126/science.aau0099

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Mysterious males

In parthenogenetic species, females produce female offspring, generally without the input of males. Given this, the production of males would seem to be a waste of resources. Grosmaire et al. report that in a particular soil nematode, males are regularly produced at a rate of about 9%. They found that the male sperm was required for egg activation, yet the sperm DNA never transmitted on to the subsequent female generation. Provided that male DNA is preferentially passed on through sibling mating, male production is evolutionarily stable.

Science, this issue p. 1210


We report the reproductive strategy of the nematode Mesorhabditis belari. This species produces only 9% males, whose sperm is necessary to fertilize and activate the eggs. However, most of the fertilized eggs develop without using the sperm DNA and produce female individuals. Only in 9% of eggs is the male DNA utilized, producing sons. We found that mixing of parental genomes only gives rise to males because the Y-bearing sperm of males are much more competent than the X-bearing sperm for penetrating the eggs. In this previously unrecognized strategy, asexual females produce few sexual males whose genes never reenter the female pool. Here, production of males is of interest only if sons are more likely to mate with their sisters. Using game theory, we show that in this context, the production of 9% males by M. belari females is an evolutionary stable strategy.

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