Diversity in the reproductive modes of females of the Rutilus alburnoides complex (Teleostei, Cyprinidae):: A way to avoid the genetic constraints of uniparentalism

Hybrid minnows collectively known as the Rutilus alburnoides complex are found throughout much of the Iberian Peninsula and include diploid and polyploid forms with female-skewed sex ratios. Previous studies have suggested that diploid and triploid females from the northern Douro Basin reproduce by hybridogenesis. The present study, which is based on experimental crosses and uses allozyme and minisatellite markers, reveals that diploid females from the Tejo Basin exhibit a different form of reproduction, transmitting the hybrid genome intact to the egg, which, upon fertilization, yields triploid progeny. Reproduction by triploid females from the southern Guadiana and Tejo basins resembles hybridogenesis in that one genome is discarded in each generation without recombination, but the remaining two homospecific genomes are not transmitted clonally. Elimination of the unmatched genome permits ready synapsis and meiosis between the homospecific genomes, and genetically distinct haploid eggs are produced ("meiotic hybridogenesis"). In some females, some sexual cells undergo an altered nonreductional meiosis, resulting in genetically diverse diploid eggs. In contrast to most hybrid vertebrate complexes, in which diploids and triploids are evolutionarily independent, in the R. alburnoides complex, there is a bidirectional movement of genes between diploid and triploid hybrids. Reproduction by the types of diploid and triploid females discussed here introduces high genotypic diversity into hybrid populations, and allows purging of deleterious genes and incorporation of beneficial mutations in the same genome, characteristics believed to be major advantages of sexual reproduction.