Evolution of hermaphroditic mating systems in animals

Abstract

Miksi jotkin eläinlajit ovat kaksineuvoisia ja toisissa lajeissa koiras- ja naarasominaisuudet ovat eri yksilöissä? Kaksineuvoiset eläimet pystyvät lisääntymään itsesiitoksella ilman kumppania tai ristisiitoksella kumppanin kanssa. Kumpi lisääntymistapa on parempi? Muun muassa näitä kysymyksiä väitöskirjassaan pohtinut Mikael Puurtinen loi tutkimuksessaan teoriaa lisääntymistapojen evoluutiosta, sekä selvitti vedessä elävän isolimakotilon (Lymnaea stagnalis) lisääntymisbiologiaa. Suomen Akatemian nimittämässä evoluutioekologian huippuyksikössä tehty tutkimus toi arvokasta tietoa populaation geneettisen monimuotoisuuden vaikutuksista populaation elinkykyyn sekä kykyyn vastustaa loistauteja.

This thesis is focused on the advantages of hermaphroditism versus separate sexes in animals, on the evolution of self- and cross-fertilization in populations of the hermaphroditic aquatic snail Lymnaea stagnalis, and on the consequences of small historical population size for reproductive output and parasite resistance of snails. Hermaphroditism has been suggested to be favoured when the availability of mating partners is limited. Many animals can however increase the availability of mating partners by searching. We found that limited mate search efficiency stabilizes hermaphroditism, and more interestingly, that high mate search efficiency causes disruptive selection on searching behaviour, leading to the evolution of males and females. In our empirical studies of Lymnaea stagnalis populations, we found that these snails typically reproduce by cross-fertilization in nature. In experimental studies we found that evolution of self-fertilization does not seem to be opposed by lower fecundity of self-fertilizing snails, nor by considerably higher early mortality or higher risk of parasitism for self-fertilized offspring. Population genetic data revealed that the studied Lymnaea stagnalis populations had different histories that could be inferred from population genetic data; some populations had been very small and had lost most of the genetic variation, while others had retained larger sizes and more genetic variation. Small historical population sizes were found to be associated with slow maturation and low fecundity of snails, suggesting that slightly harmful mutations have accumulated in small populations by genetic drift, and that mutation accumulation may endanger the long-term persistence of small populations. Snails with low genetic variability were also more likely to become infected by trematode parasites, suggesting that loss of genetic variation may threaten the survival of small populations also on short time-scales