Synaptonemal complexes in the hybridogenetic Squalius alburnoides fish complex: new insights on the gametogenesis of allopolyploids.

In the Squalius alburnoides fish complex, allotriploid females (3n = 75) reproduce mostly by meiotic hybridogenesis, producing haploid gametes by means of the elimination of the heterospecific chromosome set and recombination between the 2 homospecific genomes. A synaptonemal complexes (SCs) analysis was performed in specimens from a confined southern population (Quarteira, Portugal) to understand chromosome dynamics during gametogenesis. The comparative study between hybrid females with QAA genome composition and the parental bisexual species Squalius aradensis (2n = 50, QQ genome) evidenced: (i) that allotriploid meiocytes comprise the complete chromosome set (75 chromosomes) in prophase I, proving the heterospecific genome (Q) is only excluded after pachytene stage, and (ii) a 2-phase synaptic process where initially, exclusively homologous SCs form and the unmatched univalents remain in a bouquet conformation, followed by the establishment of extensive non-homologous SCs with multivalent associations among the later. These findings disagree with most literature concerning the meiotic process in allotriploid vertebrates, since the most accountable mechanisms (premeiotic exclusion of the unmatched chromosome set and whole genome endoduplication) were not observed.