Chromosomal Evolution in Lower Vertebrates: Sex Chromosomes in Neotropical Fishes

Abstract

Fishes exhibit the greatest diversity of species among vertebrates, offering a number of relevant models for genetic and evolutionary studies. The investigation of sex chromosome differentiation is a very active and striking research area of fish cytogenetics, as fishes represent one of the most vital model groups. Neotropical fish species show an amazing variety of sex chromosome systems, where different stages of differentiation can be found, ranging from homomorphic to highly differentiated sex chromosomes. Here, we draw attention on the impact of recent developments in molecular cytogenetic analyses that helped to elucidate many unknown questions about fish sex chromosome evolution, using excellent characiform models occurring in the Neotropical region, namely the Erythrinidae family and the Triportheus genus. While in Erythrinidae distinct XY and/or multiple XY-derived sex chromosome systems have independently evolved at least four different times, representatives of Triportheus show an opposite scenario, i.e., highly conserved ZZ/ZW system with a monophyletic origin. In both cases, recent molecular approaches, such as mapping of repetitive DNA classes, comparative genomic hybridization (CGH), and whole chromosome painting (WCP), allowed us to unmask several new features linked to the molecular composition and differentiation processes of sex chromosomes in fishes.

Molecular Analysis and Chromosome Mapping of Repetitive DNAs in the Green TerrorAndinoacara rivulatus(Cichlidae: Cichlasomatini)

“…This pattern suggests the absence of sex chromosomes, as in fish these chromosomes tend to accumulate repeated sequences partly or completely heterochromatic, [66][67][68] which could be detected as heterochromatic blocks. 69 Location of ribosomal genes has been investigated in thousands of species, making these chromosomal regions a preferred marker in comparative cytogenetics and systematic studies. 70 An.…”

Multiple evidences suggest sox2 as the main driver of a young and complex sex determining ZW/ZZ system in turbot (Scophthalmus maximus)

“…Eigenmannia have been reported (Cioffi et al, 2017), the differential region in most species where the master SDg has been identified comprehends less than 1 Mb . It is generally accepted that the evolution from a homomorphic to a heteromorphic sex chromosome pair begins with the suppression of recombination to maintain particular haplotype combinations to avoid sexual conflict (Bull 1983;Charlesworth, 1994Charlesworth, , 1996.…”

“…Fish show highly diverse SD systems, including analogous models to the XY of mammals and ZW of birds, but also more complex mechanisms involving multiple sex chromosomes. Regardless of their SD system, chromosome heteromorphisms are rare in this vertebrate group (Cioffi et al, 2017). Underlying this diversity, an important number of different SDg have been reported, both involving classical transcription factors, such as dmy (Y-specific DM-domain) (Matsuda et al, 2002) or sox3 (SRYrelated HMG-box) (Takehana et al, 2014), as well as transforming growth factorrelated genes, such as gsdf (gonadal soma-derived growth factor on the Y chromosome) (Myosho et al, 2012) or amh (anti-Mullerian hormone) (Hattori et al, 2012;Pan et al, 2019) and its receptor amhr2 (Kamiya et al, 2012), but also other unexpected genes, such the interferon-related sdY (Yano et al, 2013), and more recently, bcar1 (Bao et al, 2019) and hsd17b1 (Koyama et al, 2019), related to the steroidogenic pathway.…”

Centric Fusions behind the Karyotype Evolution of Neotropical Nannostomus Pencilfishes (Characiforme, Lebiasinidae): First Insights from a Molecular Cytogenetic Perspective

“…Moreover, the amount of cryptic and until now morphologically undistinguishable species suggests much higher species diversity (e.g., [4][5][6][7][8][9]). Fueled by these discoveries, the knowledge about the karyotype differentiation in Neotropical fishes has been rapidly growing (especially during the last few decades) and several important models for studying both sympatric and allopatric speciation, species complexes, and sex chromosome evolution have emerged [9][10][11]. As a prominent example, a remarkable cytogenetic variability has been found in the Erythrinidae family (Characiformes) and especially in Erythrinus erythrinus and Hoplias malabaricus, where several cases of multiple karyotype forms per species, high dynamics of repetitive DNA distribution, and intriguing diversity of chromosomal sex determination have been reported [10,12].…”