http://aerg.canberra.edu.au/library/sex_general/2002_Devlin_Nagahama_sex_determination_in_fish_review.pdf

Sex determination and sex differentiation in fish: an overview of genetic, physiological, and environmental influences

Although hybridization and introgression have been considered important in generation of plant diversity, their role in evolutionary diversification of animals remains unclear In this review, we reconsider the significance of introgressive hybridization in evolution and diversification of animals to determine if the generally negative assessment of these processes is warranted Unlike the situation for plants, hybrid animal taxa appear to be relatively rare This could, however, be due to negative attitudes toward hybridization and difficulty in detecting such forms Hybridization has been responsible for instantaneous creation of several unique complexes of polyploid and unisexual animals Allopolyploidy has allowed for diversification, whereas unisexual taxa have acted as conduits of gene exchange among related sexual species Many instances of diploid, bisexual taxa of hybrid origin have been put forward, but few have been carefully tested Changing attitudes toward hybrids and technological advances should allow for careful consideration of hypothesized hybrid taxa and will undoubtedly increase the number of known animal hybrid taxa

/pdf/the-role-of-hybridization-and-introgression-in-the-j5cooh114g.pdf

The role of hybridization and introgression in the diversification of animals

The Iberian minnow Leuciscus alburnoides represents a complex of diploid and polyploid forms with altered modes of reproduction. In the present paper, we review the recent data on the origin, reproductive modes, and inter-relationships of the various forms of the complex, in order to predict its evolutionary potential. The complex follows the hybrid-origin model suggested for most other asexual vertebrates. Diploid and triploid females from the southern river basins exhibit reproductive modes that cannot be conveniently placed into the categories generally recognised for these vertebrate complexes, which imply continuous shifting between forms, where genomes derived from both parental ancestors are cyclically lost, gained or replaced. Replacement of nuclear genomes allow the introduction of novel genetic material, that may compensate for the disadvantages of asexual reproduction. Contrasting with most other vertebrate complexes, L. alburnoides males are fertile and play an important role in the dynamics of the complex. Moreover, diploid hybrid males may have initiated a tetraploidization process, when a diploid clonal sperm fertilised a diploid egg. This direct route to tetraploidy by originating fish with the right constitution for normal meiosis (symmetric), may eventually lead to a new sexually reproducing polyploid species. This case-study reinforces the significance of hybridisation and polyploidy in evolution and diversification of vertebrates.

Evolution in action through hybridisation and polyploidy in an Iberian freshwater fish: a genetic review.

In so-called unisexual teleost fishes, a broad spectrum of evolutionary stages with varying amounts of sexual elements has evolved. These range from pure sperm-dependent parthenogenesis (gynogenesis) without or with different amounts of paternal leakage to hybridogenesis with hemiclonal diploid gametogenesis or genome elimination followed by meiosis (meiotic hybridogenesis). All of these phenomena are of hybrid origin.

/pdf/sperm-dependent-parthenogenesis-and-hybridogenesis-in-3d9onhk5bm.pdf

Sperm-Dependent Parthenogenesis and Hybridogenesis in Teleost Fishes

The cyprinid genus Barbus, with more than 800 nominal species, is an apparently polyphyletic assemblage to which a number of unrelated species, groups and/or assemblages have been assigned. It includes species that exhibit three different ploidy levels: diploid, tetraploid and hexaploid. Several lineages of the family Cyprinidae constitute a major component of the African freshwater ichthyofauna, having about 500 species, and fishes assigned to the genus 'Barbus' have the most species on the continent. We used complete sequences of the mitochondrial cytochrome b gene in order to infer phylogenetic relationships between diploid, tetraploid and hexaploid species of 'Barbus' occurring in southern Africa, the only region where representatives of all of the three ploidy levels occur. The results indicate that most of the lineages are incorrectly classified in the genus 'Barbus'. The southern African tetraploids probably originated from southern African diploids. They constitute a monophyletic group distinct from tetraploids occurring in the Euro-Mediterranean region (Barbus sensu stricto). The 'small' African diploid species seem to be paraphyletic, while the 'large' African hexaploid barbs species are of a single, recent origin and form a monophyletic group. The evidence of multiple, independent origins of polyploidy occurring in the African cyprinine cyprinids thus provides a significant contribution to the knowledge on the systematic diversity of these fishes, and warrants a thorough taxonomic reorganization of the genus.

V.P. Vasil’ev

Papers

Evolution of a diploid-triploid-tetraploid complex in fishes of the genus Cobitis (Pisces, Cobitidae)

The problem of reticular speciation in vertebrates: the diploid-triploid-tetraploid complex in the genus Cobitis (Cobitidae). I. Diploid species