Early Stages of XY Sex Chromosomes Differentiation in the Fish Hoplias malabaricus (Characiformes, Erythrinidae) Revealed by DNA Repeats Accumulation

Abstract

Background: Species with ‘young’ or nascent sex chromosomes provide unique opportunities to understand early evolutionary mechanisms (e.g. accumulation of repetitive sequences, cessation of recombination and gene loss) that drive the evolution of sex chromosomes. Among vertebrates, fishes exhibit highly diverse and a wide spectrum of sex-determining mechanisms and sex chromosomes, ranging from cryptic to highly differentiated ones, as well as, from simple to multiple sex chromosome systems. Such variability in sex chromosome morphology and composition not only exists within closely related taxa, but often within races/populations of the same species. Inside this context, the wolf fish Hoplias malabaricus offers opportunity to investigate the evolution of morphologically variable sex chromosomes within a species complex, as homomorphic to highly differentiated sex chromosome systems occur among its different karyomorphs.Materials & Methods: To discover various evolutionary stages of sex chromosomes and to compare their sequence composition among the wolf fish´s karyomorphs, we applied multipronged molecular cytogenetic approaches, including C-banding, repetitive DNAs mapping, Comparative Genomic Hybridization (CGH) and Whole Chromosomal Painting (WCP). Our study was able to characterize a cryptically differentiated XX/XY sex chromosome system in the karyomorph F of this species.Conclusion: The Y chromosome was clearly identified by an interstitial heterochromatic block on the short arms, primarily composed of microsatellite motifs and retrotransposons. Additionally, CGH also identified a male specific chromosome region in the same chromosomal location, implying that the accumulation of these repeats may have initiated the Y chromosome differentiation, as well as played a critical role towards the evolution and differentiation of sex chromosomes in various karyomorphs of this species.

Chromosomal Evolution in Lower Vertebrates: Sex Chromosomes in Neotropical Fishes

“…From the cytogenetic standpoint, seven distinct karyotype forms or karyomorphs (A–G) in H. malabaricus and four (A–D) in E. erythrinus have already been identified, respectively, based on major differences in diploid chromosome numbers (2n), chromosome morphology, and sex chromosomes [ 44 ]. Studies employing WCP, CGH, and repetitive DNA distribution patterns have highlighted the differentiation plasticity of the erythrinid sex chromosome systems [ 25 , 45 , 46 , 47 ]. More specifically, it was demonstrated that sex chromosomes could emerge via independent pathways, following distinct patterns of differentiation even within the same type of system and among closely related karyomorphs ( Figure 1 ).…”

“…Karyomorph F (2n = 40, for both sexes) also exhibits a nascent sex chromosome system of XX/XY type [ 45 ]. The male-specific content was shown as a prominent interstitial heterochromatic block on the short (p) arms of the large metacentric Y chromosome, also accumulated with several microsatellite motifs and retrotransposons (RTEs), that are absent on the X chromosome [ 45 ].…”

“…Karyomorph F (2n = 40, for both sexes) also exhibits a nascent sex chromosome system of XX/XY type [ 45 ]. The male-specific content was shown as a prominent interstitial heterochromatic block on the short (p) arms of the large metacentric Y chromosome, also accumulated with several microsatellite motifs and retrotransposons (RTEs), that are absent on the X chromosome [ 45 ]. By contrast, karyomorph G (2n = 40 in females/ 41 in males) presents an XX/XY 1 Y 2 neo-sex chromosomes, where the unusual acrocentric Y 1 chromosome carries the male-specific region that is also enriched by several types of repetitive DNAs [ 47 ].…”

Contributions to the cytogenetics of the Neotropical fish fauna

“…The modern molecular cytogenetics was a key step for understanding the evolutionary process of the sex chromosomes among fishes. This way, the significative role of several classes of repetitive DNAs in the differentiation path of the sex pair, both at its inicial stage (Cioffi and Bertollo 2010; Freitas et al in press) or more advanced ones (Cioffi et al 2010, 2011a, b, 2012b; Yano et al 2014a, b), was clearly highlighted. Notably, whole chromosome painting (WCP) and comparative genomic hybridization (CGH) were able to demonstrate that fish sex chromosomes can have an independent origin even among closely related species (Cioffi et al 2011c, d; 2013) or, alternatively, a common origin within particular monophyletic groups (Yano et al 2016).…”

“…A larger number of Neotropical fish species with well differentiated sex chromosomes occur in comparison to other world regions (Moreira-Filho et al 1993), carrying simple (ZZ/ZW, XX/XY) and multiple (X 1 X 1 X 2 X 2 /X 1 X 2 Y, XX/XY 1 Y 2 , ZZ/ZW 1 W 2 ) sex chromosome systems (Centofante et al 2002; Cioffi et al 2012b), in addition to some others disclosing a nascent or early stage of differentiation (Cioffi and Bertollo 2010; Freitas et al in press). Usually, sex chromosomes occur as a particular feature for some species within a specific fish group, as exemplified in the Erythrinidae, Parodontidae, Anostomidae and Crenuchidae families (Galetti Jr. et al 1981, 1995; Moreira-Filho et al 1985, 1993; Molina et al 1998; Centofante et al 2001, 2003; Bertollo et al 2000; Bertollo 2007; Vicari et al 2008; Cioffi et al 2013).…”