ABSTRACT
There are 80 unisexual biotypes among vertebrates. However, due to their amenability to rearing and experimentation (see Pandian, 2011), a volume of literature is available on eight distinct unisexual fi shes (Lamatsch and Stock, 2009; Pandian, 2010, 2011). Having arisen as hybrids on multiple occasions, each unisexual fi sh species has explored its own mode and cycle of reproduction (Fig. 21). To avoid the onslaught of mutational meltdown and/or Muller’s ratchet, the unisexual fi shes have devised different escape mechanisms: (i) paternal leakage, (ii) paternal genome replacement by hybridogenesis, and (iii) paternal genome additions, which produce triploids and tetraploids. Of these, paternal genome additions occur in seven species, hybridogenesis in three species (Table 19) but paternal leakage is limited to Poecilia formosa and Carassius auratus. Hence the paternal genome addition is the preferred escape mechanism of the majority of unisexual fi shes. Among the ploids, triploids occur in seven species but tetraploids are limited to three or four species. With the ubiquitous presence of triploids among the unisexuals, gametogenic function is impaired by the presence of three sets of homologous/heterologous chromosomes creating incompatibilities during gametogenesis (Pandian, 2011). However, publications describing the expression and inheritance of paternal genes or genome(s) in the unisexual progenies are limited. Even more limited are the publications that trace the cause(s) for the failure of spermatogenesis and describe the functioning of endocrine mechanism of gametogenesis. To point out the need for critical inputs in this area, this chapter has a few repetitions of some sections from earlier volumes in this series.
