Sex-shift surprise

Abstract

Populations of males and hermaphrodites appear to have a surprising evolutionary endurance in one group of freshwater crustaceans. Nigel Williams reports. Populations of males and hermaphrodites appear to have a surprising evolutionary endurance in one group of freshwater crustaceans. Nigel Williams reports. While most individuals in sexual species divide into either males or females, evolutionists face a challenge in explaining those species that do not conform to this type: hermaphrodites are the norm in some species and others show a more exotic mix of individuals with varying sexual identities. How these came about and what light they may throw on the evolution of separate sexes has been an intriguing question for both field and theoretical biologists. In a new study led by Stephen Weeks at the University of Akron, Ohio, reported in the Proceedings of the Royal Society (published online), researchers believe that one sexual system — males and hermaphrodites, that theoreticians believe to be evolutionary unstable — may actually be an ancient and enduring lifestyle in one group of freshwater shrimps. When evolution favours a shift from one reproductive mode to another, three temporary forms of reproduction are thought to accompany the transition: a mixture of hermaphrodites, males and females (trioecy), hermaphrodites and females (gynodioecy), or hermaphrodites and males (androdioecy). Of these, many researchers believe trioecy to be highly unstable, and thus, gynodioecy or androdioecy are predicted to be the two likely transitionary reproductive modes. Field researchers have found gynodioecy to be more common than androdioecy, which is suggested by theoretical studies of breeding system evolution. Researchers have developed two categories of models that might lead to the persistence of androdioecy in species in which this breeding system has been documented. But “because the conditions for the stable maintenance of males with hermaphrodites are stringent in these models, androdioecy is predicted to be short-lived,” the researchers say. To date, no multi-species, higher-level taxon has been described as being composed of only androdioecious species, again suggesting that androdioecy is a relatively recent and transient reproductive development in the various taxa in which it occurs. One well-described androdioecious system is found in the primitive crustacean class Branchipodia, specifically in the clam shrimp Eulimnadia texana Populations of these shrimps comprise males and self-compatible hermaphrodites. Previous studies have suggested that the transition to androdioecy in the clam shrimp may have been a rather recent event. But, because of the ancient origins of the Branchiopodia, other have suggested a more ancient origin. In the new study, the team has studied the reproductive strategies of a number of species of shrimp within the genus Eulimnadia from all continents where they occur. The actual number of species is currently debated but thought to be between 28 and 44. The team report data on 11 species and find that nine showed definitive evidence of androdioecy. “Because these species represent a random subset of the 28–44 species of Eulimnadia and are distributed across all continents that contain the shrimp, the most parsimonious explanation at present for the preponderance of this breeding system in Eulimnadia is that it arose at or before the origin of this genus,” the authors report. As widespread recent dispersal of these shrimps appears unlikely, the researchers have compared populations with the history of continental movements that may have separated previously closely associated populations. Such ancient separations, the authors believe, have meant that these clam shrimp have retained androdioecy through multiple speciation events and across tens to hundreds of millions of years, suggesting “that in this genus, androdioecy has been a highly successful and important mode of reproduction.” These results throw up fresh fuel for the theoreticians. “Any truly comprehensive model seeking to delineate the benefits of separation of the sexes relative to hermaphroditism must now explain the long-lived coexistence of males with hermaphrodites in the Eulimnadia crustacea,” the authors believe.