Comparative Cytogenetics and Neo-Y Formation in Small-Sized Fish Species of the Genus Pyrrhulina (Characiformes, Lebiasinidae)

Abstract

Although fishes have traditionally been the subject of comparative evolutionary studies, few reports have concentrated on the application of multipronged modern molecular cytogenetic techniques (such as comparative genomic hybridization = CGH and whole chromosome painting = WCP) to analyze deeper the karyotype evolution of specific groups, especially the historically neglected small-sized ones. Representatives of the family Lebiasinidae (Characiformes) are a notable example, where only a few cytogenetic investigations have been conducted thus far. Here, we aim to elucidate the evolutionary processes behind the karyotype differentiation of Pyrrhulina species on a finer-scale cytogenetic level. To achieve this, we applied C-banding, repetitive DNA mapping, CGH and WCP in Pyrrhulina semifasciata and P. brevis . Our results showed 2n = 42 in both sexes of P. brevis , while the difference in 2n between male and female in P. semifasciata (♂41/♀42) stands out due to the presence of a multiple X 1 X 2 Y sex chromosome system, until now undetected in this family. As a remarkable common feature, multiple 18S and 5S rDNA sites are present, with an occasional synteny or tandem-repeat amplification. Male- vs .-female CGH experiments in P. semifasciata highlighted the accumulation of male-enriched repetitive sequences in the pericentromeric region of the Y chromosome. Inter-specific CGH experiments evidenced a divergence between both species’ genomes based on the presence of several species-specific signals, highlighting their inner genomic diversity. WCP with the P . semifasciata -derived Y (PSEMI-Y) probe painted not only the entire metacentric Y chromosome in males but also the X 1 and X 2 chromosomes in both male and female chromosomes of P. semifasciata. In the cross-species experiments, the PSEMI-Y probe painted four acrocentric chromosomes in both males and females of the other tested Pyrrhulina species. In summary, our results show that both intra- and interchromosomal rearrangements together with the dynamics of repetitive DNA significantly contributed to the karyotype divergence among Pyrrhulina species, possibly promoted by specific populational and ecological traits and accompanied in one species by the origin of neo-sex chromosomes. The present results suggest how particular evolutionary scenarios found in fish species can help to clarify several issues related to genome organization and the karyotype evolution of vertebrates in general.

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

“…Altogether with Ctenoluciidae [37], this pattern can be considered as symplesiomorphy for both families. Although 5S rDNA displays a more dynamic evolution, with both terminal and interstitial signals among lebiasinids ( [19][20][21][22], this study), it is noteworthy that N. unifasciatus underwent structural chromosome rearrangements involving both 18S and 5S rDNA loci, which have led to a derived pattern of rDNA distribution in this species. It is a rather expected scenario for N. unifasciatus, since this species exhibits the lowest 2n among Lebiasinidae fishes (2n = 22) and hence it may be speculated that the proximal 18S and 5S rDNA sites found in N. unifasciatus might rather represent hallmarks of fusion, suggesting the probable direction of chromosome change in this genus.…”

“…Nevertheless, a steadily growing body of information on karyotype characteristics in Lebiasinidae has been generated within recent years, using both conventional and molecular cytogenetic techniques, bringing new important pieces into the puzzle of lebiasinid karyotype differentiation and its underlying evolutionary mechanisms. More specifically, high rate of repetitive DNA dynamics and the occasional emergence of neo-sex chromosomes were found among four Pyrrhulina taxa; one of them may represent a new, yet undescribed species [19,20]. Furthermore, contrasting patterns of repetitive DNA content and distribution as well as a putative nascent sex chromosome system were also reported for Lebiasina species, supporting at the same time relationships between the Lebiasinidae and Ctenoluciidae families [21].…”

“…In the case of rDNAs, this may be possibly linked with the susceptibility of these tandemly repeated clusters to double-stranded DNA breaks, perhaps resulting from (1) a frequent rRNA transcription and thus break-prone R-loop emergence, (2) intermingling of NOR (Nuclear Organizer Region)-bearing chromosomes in the interphase nucleus, or (3) possible association of rDNA-bearing sites during the meiotic prophase I [42][43][44][45][46][47][48]. With a few exceptions, the terminal position of the 18S rDNA loci on chromosomes appears to be a common feature for all Lebiasinidae genera analyzed up to now (i.e., Nannostomus, Pyrrhulina, Lebiasina, and Copeina) ( [19][20][21][22], this study). Altogether with Ctenoluciidae [37], this pattern can be considered as symplesiomorphy for both families.…”

“…By altering gene expression or by joining previously unlinked genetic material together, for instance, they might facilitate the emergence of evolutionarily advanced (e.g., locally adapted) sub-populations of a given species, thus contributing to diversification [69,101]. In addition, they might also be linked to the emergence of novel sex chromosome systems, such as that recently found in the lebiasinid genus Pyrrhulina [20]. Lastly, although additional detailed cytogenetic studies are still needed on a wider taxonomic scale, the present data reinforced the assumption that chromosomal fusions were important drivers of the karyotype evolution in the Neotropical family Lebiasinidae and, especially, in the pencil fishes of the genus Nannostomus.…”

“…In contrast to relative stability of the 2n in Copeina, Lebiasina and Pyrrhulina species karyotyped to date [16][17][18][19][20][21][22], representatives of Copella and Nannostomus display remarkable karyotype variability [16,18]. Indeed, even from limited karyotype data, it can be inferred that Nannostomus exhibits a wide range of 2n, from 22 (in N. unifasciatus) to 46 (in N. trifasciatus) [16,18,23], suggesting an important role of Robertsonian rearrangements in its karyotype differentiation.…”