Population ecology and sexual preferences in the mating complex of the unisexual Amazon molly Poecilia formosa (GIRARD, 1859)

The Amazon molly Poecilia formosa is a gynogenetic all-female ovoviviparous fish. Gynogenesis is a special form of parthenogenesis; sperm is needed to trigger embryogenesis. Males do not contribute to the genome of ameiotically produced all-female offspring. In this unique mating system, asexually reproducing females have to rely on heterospecific matings with males of two closely related sexual host species, P. latipinna and P. mexicana. This asexual / sexual species complex is a unique model system to study the stability of coexistence of closely related bisexual and unisexual species. In mixed shoals of a sexual host species and the sexual-parasite Poecilia formosa, sperm donating males mate with conspecific sexual females and heterospecific asexual females. Obviously, males benefit from mating with conspecific females, whereas mating with the unisexual P. formosa does not increase a male’s fitness. Therefore, males should discriminate between the two types of females. Under perfect mate discrimination, the Amazon molly P. formosa would disappear. On the other hand, asexually reproducing females theoretically have a faster population growth than sexually reproducing females because they do not have to incur the cost of producing males. This advantage of asexual reproduction would lead to increasingly high proportions of asexual Amazon mollies (P. formosa) in mixed populations. By outcompeting its sexual host species, the sexual-parasite P. formosa would also disappear. The aim of this thesis is to study the coexistence and stability of the asexual / sexual mating complex of P. formosa and how it can be maintained. Therefore, I studied the ecological situation and patterns of coexistence in several mixed populations in the field and conducted experiments on mate choice behaviour in the asexual / sexual species complex of the Amazon molly P. formosa. Outline of this thesis This thesis is organized in seven chapters. The technical details of each study can be found in the specific chapters. Chapter one is an introduction and thus written in the form of an essay. Chapter 2 -5 are structured in summary, introduction, methods, results, and discussion. Chapter six is a short communication and chapter seven a general discussion and conclusion. This outline is a very general introduction to the concepts of the research questions. Chapter 1 presents a general introduction to the evolution of sex, advantages and disadvantages of asexual reproduction, and persistence of the gynogenetic Amazon molly, Poecilia formosa. Some existing general hypotheses on advantages and disadvantages of asexual reproduction are discussed with particular reference to those models that may also apply to sperm dependent unisexual fishes. In chapter 2, an ecological field study is presented and discussed. The aim of this study was to investigate possible niche differentiation and population dynamics in this species-complex. Therefore, the ecology of the coexistence of the Amazon molly and the closely related sperm donating bisexual species P. latipinna has been studied in several sympatric populations in