Servizio Comunitario di Informazione in materia di Ricerca e Sviluppo - CORDIS

Final Activity Report Summary - SEXASEX (Sex to asex: a case study on transitions and coexsistence between sexual and asexual reproduction)

Sex is the queen of problems in evolutionary biology. Although asexual reproduction offers several clear short-term advantages, ruling evolutionary theory dictates that the absence of sex over long time frames, will direct clones into evolutionary dead ends. Several animal and plant groups nevertheless show a large incidence of asexual reproduction and some lineages might have been fully asexual for many millions of year. The phenomenon of sexual or asexual reproduction is thus at present one of the hot topics in evolutionary biology. Ostracoda are an excellent model group to address this paradox, as several reproductive strategies occur (fully sexual, mixed and fully asexual), while the abundant fossil record allows for the study of effects of reproductive modes in real time.

SEXASEX provided training and transfer of knowledge for a total of 381 man months. Fourteen network researchers (4 male, 10 female) have received training in 9 institutions and countries (including 2 partner countries, 2 less-favoured regions). Career development plans, individual tutoring and secondments were supplemented by participation in courses and external workshops. Network-wide training fully exploited the consortium's potential in seven training workshops for network researchers as well as for external participants. Management and communication (verbal, written) were developed as auxiliary tools; gender awareness was raised in various network-wide meetings.

Theoretical and empirical research trained the network researchers in 15 major methodologies, including novel genomic and cytogenetic approaches, molecular phylogeny, behavioural studies, GIS analyses and theoretical modelling. With this multidisciplinary approach, we have shown that the model species Eucypris virens is actually a species cluster, in which phylogenetic and coalescent analyses demonstrated the presence of more than 30 discrete clusters, or 'cryptic species'. Some are fully sexual, some fully asexual and a few are mixed. The presence of fully asexual, deep-rooted clusters seems to indicate that no continued interaction between asex and sex is necessary for the survival of asexual lineages. However, there are suggestions that sexual reproduction may have continued until recently in these deep lineages.

There appears to be no Meselson effect in E. virens. So, either, no lineages have persisted long enough to allow for intra-individual allelic divergence, or homogenising mechanisms, such as (mitotic) gene conversion, have countered this effect. An apparent absence of parasites in ostracods seems to rule out the importance of the Red Queen for the existence and persistence of sexual reproduction, at least in E. virens.

There are both diploid (N=24) and triploid (N=36) individuals. Sexuals are exclusively diploid, asexual can be either. It is assumed that most asexual triploids originate from hybridisation events between a male and an asexual female with diploid eggs, resulting in triploid zygotes, and some of these hybridisation events have been documented using DNA sequence data. Most diploids occur around the Mediterranean, while triploids occur everywhere. Although the pattern is not absolute, it shows a clear link to that of geographic parthenogenesis, where asexuals occur everywhere, but sexual reproduction is limited to the Mediterranean.

A wide range of environmental variables, both biotic and abiotic, have been monitored to determine why sexuals occur only in the south (geographic parthenogenesis), and especially why both reproductive modes can co-occur there, sometimes in the same pond. Most variables had no apparent effect on differential fitness, only small but significant differences in hatching success in different latitudinal circumstances were detected. Historical contingency, for example post-glacial colonisation of Western Europe from the Mediterranean, might thus, more than present day ecological drivers, be responsible for these geographical patterns.

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