Abstract
In Europe, two bisexual fish species, Cobitis taenia (TT) and Cobitis elongatoides (EE), hybridized, producing diploid and polyploid clonal lineages. This study compared, for the first time, embryonic development, hatching success, sex ratio, body size and as well as reproductive ability based on the gonad histology of F1 reciprocal diploid hybrids (TE, ET) of both species. Hybrid F1 progeny showed the same proper pattern of embryonic and larval development. Among TE and ET offspring, slightly more females and males, respectively, occurred, but sex parity among 18-month-old hybrids was observed. Two- and three-year-old F1 hybrid females were mature, possessing all stages of oogenesis in their ovaries. Females (TE) back-crossed with C. taenia males had properly developing progeny. In testes of two- and three-year-old F1 hybrids, only early stages of spermatogenesis and pyknotic cells indicating the degeneration process were observed, but they exhibited an external feature (lamina circularis) of maturation. The results were confirmed by the structure of gonads that two sexual species, C. taenia and C. elongatoides, hybridize, producing F1 progeny of sterile males and fertile females. Hybrid females may participate in subsequent steps of speciation via hybridization and polyploidy or, on the contrary, represent an element of new species isolation.
Highlights
Hybridization is a potentially powerful mechanism of diversification among vertebrates and can lead to adaptation through the creation of novel genotypes and morphologies (Hayden et al, 2010)
The loaches of Cobitis in Central Europe occur mainly in mixed diploid–polyploid populations, in which individuals of spined loach C. taenia Linnaeus, 1758 and/or Danubian spined loach C. elongatoides Bacescu & Maier, 1969 co-exist with their diploid and polyploid hybrids (Boron, 2003; Choleva & Janko, 2013; Choleva et al, 2014). Both of these species are small-sized, bottom dwelling fish. They are classified in the IUCN Red List Categories of Least Concern (LC) (IUCN, 2017) as an endangered and protected species in some European countries, mainly due to a decrease in the number of favorable habitats as well as a domination of allotriploid Cobitis females in most populations
An average of 329 eggs and 713 eggs were obtained from C. taenia females and C. elongatoides female, respectively
Summary
Hybridization is a potentially powerful mechanism of diversification among vertebrates and can lead to adaptation through the creation of novel genotypes and morphologies (Hayden et al, 2010). The loaches of Cobitis in Central Europe occur mainly in mixed diploid–polyploid populations, in which individuals of spined loach C. taenia Linnaeus, 1758 and/or Danubian spined loach C. elongatoides Bacescu & Maier, 1969 co-exist with their diploid and polyploid hybrids (Boron, 2003; Choleva & Janko, 2013; Choleva et al, 2014). Both of these species are small-sized, bottom dwelling fish. Among 26 karyologically recognized Cobitis populations distributed in Poland, only five were composed of exclusively C. taenia and only one was composed of exclusively C. elongatoides individuals (Boronet al., unpublished)
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