Diploid-Triploid Mosaics Among Unisexual Hybrids of the Minnows Phoxinus eos and Phoxinus neogaeus

Abstract

Diploid-triploid mosaics are rarely found in vertebrates, and until now they were known to be common in only two vertebrate species complexes. Here we report that diploid-triploid mosaics are widespread among unisexual hybrids of the minnows Phoxinus eos and Phoxinus neogaeus, a complex already known to contain diploid and triploid forms. Using chromosome counts and flow cytometry, we show that the mosaics occur throughout the known range of the unisexuals and are abundant in many of these natural populations. The mosaics are highly heterogeneous, showing individual variation in the ratio of diploid to triploid cells, and as a group they appear to form a continuum between the pure diploid and triploid forms. Tissue-graft analysis shows that the third genome present in the triploid cells of a mosaic is expressed, because grafts made from the mosaics show an incidence of rejection intermediate between that of the diploid (clonal) and triploid (nonclonal) biotypes.

Contrasting views over a hybrid complex: Between speciation and evolutionary “dead-end”

“…Blood was drawn from the caudal vein and the ploidy of the specimens was determined by flow cytometry as described in Dawley and Goddard (1988).…”

Analysis of potential factors allowing coexistence in a sexual/asexual minnow complex

“…Hybrids (including diploids, triploids and mosaics) have been generally found to be as abundant as "pure" parentals (though generally including cybrids) at several locations (Joswiak et al 1985;Dawley et al 1987;Goddard et al 1989;Doeringsfeld et al 2004;Schlosser 2007, Mee et al 2012). When triploids are distinguished from mosaics, the natural triploid abundance rate (of all dace at a location) has been found to range between 9 % (Doeringsfeld et al 2004) and 18 % (Dawley and Goddard 1988;Goddard et al 1989). Mosaic abundance ranges from 5 to 15 % (Dawley and Goddard 1988;Goddard et al 1989;Doeringsfeld et al 2004).…”

“…When triploids are distinguished from mosaics, the natural triploid abundance rate (of all dace at a location) has been found to range between 9 % (Doeringsfeld et al 2004) and 18 % (Dawley and Goddard 1988;Goddard et al 1989). Mosaic abundance ranges from 5 to 15 % (Dawley and Goddard 1988;Goddard et al 1989;Doeringsfeld et al 2004). The natural hybrid abundance ranges from 0 to 100 % per locality (Binet and Angers 2005;Mee and Taylor 2012), with most locations in the northern portion of the range dominated by cybrids (Mee and Taylor 2012).…”

Cryptic clonal lineages and genetic diversity in the loach Misgurnus anguillicaudatus (Teleostei: Cobitidae) inferred from nuclear and mitochondrial DNA analyses

“…Natural triploidy and asexual development due to unreduced oogenesis have often been reported in the hybrids of Poecilia (Monaco et al 1984), Menidia (Echelle and Mosier 1982;Echelle et al 1983), Phoxinus (Dawley et al 1987; Dawley and Goddard 1988;Goddard and Dawley 1990; Goddard and Schultz 1993;Goddard et al 1998), Poeciliospsis (Quatro 1992a, b), Cobitis (Vasil'ev et al 1989;Janko et al 2003Janko et al , 2005Kitagawa et al 2003Kitagawa et al , 2005, Oryzias (Sakaizumi et al 1993;Shimizu et al 2000), and others . Therefore, atypical reproduction and polyploidy are tightly linked to a hybridization event between two or more different bisexually reproducing species, races, and/or populations (Dawley 1989;Vrijenhoek 1989).…”