Evolutionary Relationships and Cytotaxonomy Considerations in the GenusPyrrhulina(Characiformes, Lebiasinidae)

Abstract

Although fishes exhibit the greatest biodiversity among the vertebrates, a large percentage of this fauna is still underexplored on evolutionary cytogenetic questions, particularly the miniature species. The Lebiasinidae family is a particular example for such case. This study is the first one presenting differential cytogenetic methods, such as C-banding, repetitive DNAs mapping, comparative genomic hybridization (CGH), and whole chromosome painting in lebiasinid species. Pyrrhulina australis and Pyrrhulina aff. australis were deeply investigated concerning their chromosomal patterns and evolutionary relationships. These species have a very similar morphology, but they can be distinguished by a longitudinal midlateral faintly dark stripe exclusive for Pyrrhulina aff. australis. Both species presented 2n = 40 chromosomes (4st +36a), without heteromorphic sex chromosomes. However, despite their morphological and karyotype resemblance, it was evidenced that both species have already gone through a significant genomic divergence, thus corresponding to distinct evolutionary units. Furthermore, to give additional support to some proposals on evolutionary relationship among Lebiasinidae with other fish families, a chromosomal comparative approach with Erythrinus erythrinus, a representative species of the Erythrinidae family, was also performed. In addition to have similar karyotype structure, mainly composed by acrocentric chromosomes, both species share uncommon genomic similarities, such as (i) syntenic location of 5S and 18S rDNA sequences; (ii) huge dispersion of multiple 5S rDNA sites in the karyotypes; and (iii) complex association between 5S rDNA and Rex3 elements. CGH experiments, despite reinforcing some shared genomic homologies, also highlighted that both Pyrrhulina and Erythrinus have a range of nonoverlapping species-specific signals. The overall chromosomal data proved to be effective markers for the cytotaxonomy and evolutionary process among Lebiasinidae fishes.

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

“…While some of these sites may appear to be orthologous among the species under study, the frequently high dynamics of these repetitive DNA classes do not allow us to make certain conclusions without additional data (for an exemplary study, see Milhomem et al [95]). Nonetheless, in addition to the fact that some rDNA sites were clearly involved in Robertsonian fusions (as mentioned above), it may be inferred that like some other related lebiasinids [19], Nannostomus species do not show a substantial level of intrachromosomal dynamics that could be detected by the markers selected by us. This inference is further supported by the low to moderate amount of constitutive heterochromatin revealed by C-banding.…”

“…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.…”

“…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.…”