Sex Determination in the Squalius alburnoides Complex: An Initial Characterization of Sex Cascade Elements in the Context of a Hybrid Polyploid Genome

Abstract

BackgroundSex determination processes vary widely among different vertebrate taxa, but no group offers as much diversity for the study of the evolution of sex determination as teleost fish. However, the knowledge about sex determination gene cascades is scarce in this species-rich group and further difficulties arise when considering hybrid fish taxa, in which mechanisms exhibited by parental species are often disrupted. Even though hybridisation is frequent among teleosts, gene based approaches on sex determination have seldom been conducted in hybrid fish. The hybrid polyploid complex of Squalius alburnoides was used as a model to address this question.Methodology/Principal FindingsWe have initiated the isolation and characterization of regulatory elements (dmrt1, wt1, dax1 and figla) potentially involved in sex determination in S. alburnoides and in the parental species S. pyrenaicus and analysed their expression patterns by in situ hybridisation. In adults, an overall conservation in the cellular localization of the gene transcripts was observed between the hybrids and parental species. Some novel features emerged, such as dmrt1 expression in adult ovaries, and the non-dimorphic expression of figla, an ovarian marker in other species, in gonads of both sexes in S. alburnoides and S. pyrenaicus. The potential contribution of each gene to the sex determination process was assessed based on the timing and location of expression. Dmrt1 and wt1 transcripts were found at early stages of male development in S. alburnoides and are most likely implicated in the process of gonad development.Conclusions/SignificanceFor the first time in the study of this hybrid complex, it was possible to directly compare the gene expression patterns between the bisexual parental species and the various hybrid forms, for an extended set of genes. The contribution of these genes to gonad integrity maintenance and functionality is apparently unaltered in the hybrids, suggesting that no abrupt shifts in gene expression occurred as a result of hybridisation.

Meiosis and Its Deviations in Polyploid Animals

“…1 D) The Squalius alburnoides complex, an Iberian cyprinid complex [see also Collares-Perreira et al, this issue] previously known as Leuciscus, Rutilus or Tropidophoxinellus, arose by hybridization involving females of Squalius pyrenaicus (P genome) and a species closely related to Anaecypris hispanica , which is responsible for its A genome [Pala et al, 2009]. In general, each S .…”

Evidence for Hermaphroditism in the <i>Squalius alburnoides</i> Allopolyploid Fish Complex

“…The S. alburnoides hybrid complex (Cyprinidae) has been used as a model to study a variety of topics from polyploidy [Gromicho et al, 2006] to alternative reproductive strategies [Alves et al, 1998], genome interactions [Pala et al, 2008a], and sex determination and differentiation [Pala et al, 2008b[Pala et al, , 2009. However, key questions concerning reproductive development, sexual differentiation, pronounced female sex bias and the curious allmale lineage phenomenon are still without answers, and so, hermaphroditism could play an important role in some of these many unsolved cases.…”

“…Finally, despite the fact that the molecular genetic mechanisms and the regulatory networks for sex determination have only recently started to be investigated in S. alburnoides [Pala et al, 2008b[Pala et al, , 2009, there are indications of sexual lability and environmental influence. Indeed, comparative studies on sex determination in distinct fish groups, revealed that the molecular mechanisms for sex determination can be temperaturedependent, even in species with a firm genotypic sex determination [reviewed in Ospina-Alvarez and Piferrer, 2008].…”

The evolutionary history of the allopolyploid Squalius alburnoides (Cyprinidae) complex in the northern Iberian Peninsula

“…However, it is not known how sex is determined in S. alburnoides. Previous studies indicated that the mechanism of sex determination cannot be fully explained either by female or male heterogamety (Alves et al, 1998(Alves et al, , 1999 apparently being strictly a genetic mechanism (Pala et al, 2009). It seems that sex ratio in triploid forms is influenced by other Squalius genomes (C and P), either as a consequence of lower fitness of the AA, CAA/PAA males or CCA/PPA females and/or by the mechanism of sex determination.…”