Heterochromatin and microsatellites detection in karyotypes of four sea turtle species: Interspecific chromosomal differences

Integrating morphological, molecular and cytogenetic data for F2 sea turtle hybrids diagnosis revealed balanced chromosomal sets

Hybridization could be considered part of the evolutionary history of many species. The hybridization among sea turtle species on the Brazilian coast is atypical and occurs where nesting areas and reproductive seasons overlap. Integrated analysis of morphology and genetics is still scarce, and there is no evidence of the parental chromosome set distribution in sea turtle interspecific hybrids. In this study, chromosome markers previously established for pure sea turtle species were combined with morphological and molecular analyses aiming to recognize genetic composition and chromosome sets in possible interspecific hybrids initially identified by mixed morphology. The data showed that one hybrid could be an F2 individual among Caretta caretta × Eretmochelys imbricata × Chelonia mydas, and another is resulting from backcross between C. caretta × Lepidochelys olivacea. Native alleles of different parental lineages were reported in the hybrids, and, despite this, it was verified that the hybrid chromosome sets were still balanced. Thus, how sea turtle hybridism can affect genetic features in the long term is a concern, as the implications of the crossing-over in hybrid chromosomal sets and the effects on genetic function are still unpredictable.

Enriched tandemly repeats in chromosomal fusion points of Rineloricaria latirostris (Boulenger, 1900) (Siluriformes: Loricariidae)

Cytogenetic data showed the enrichment of repetitive DNAs in chromosomal rearrangement points between closely related species in armored catfishes. Still, few studies integrated cytogenetic and genomic data aiming to identify their prone-to-break DNA sites. Here, we aimed to obtain the repetitive fraction in Rineloricaria latirostris to recognize the microsatellite and homopolymers flanking the regions previously described as chromosomal fusion points. The results indicated that repetitive DNAs in R. latirostris are predominantly DNA transposons, and considering the microsatellite and homopolymers, A/T-rich expansions were the most abundant. The in situ localization demonstrated the A/T-rich repetitive sequences are scattered on the chromosomes, while A/G-rich microsatellites units were accumulated in some regions. The DNA transposon hAT, the 5S rDNA, and 45S rDNA (previously identified in Robertsonian fusion points in R. latirostris) are clusterized with some microsatellites, especially (CA)n, (GA)n, and poly-A, which also are enriched in regions of chromosomal fusions. Our findings demonstrated that repetitive sequences such as rDNAs, hAT transposon, and microsatellite units flank probable evolutionary breakpoint regions in R. latirostris. However, due to the sequence unit homologies in different chromosomal sites, these repeat DNAs only may have facilitated chromosome fusion events in R. latirostris rather than work as a double-strand breakpoint site.

Why Do Some Vertebrates Have Microchromosomes?

With more than 70,000 living species, vertebrates have a huge impact on the field of biology and research, including karyotype evolution. One prominent aspect of many vertebrate karyotypes is the enigmatic occurrence of tiny and often cytogenetically indistinguishable microchromosomes, which possess distinctive features compared to macrochromosomes. Why certain vertebrate species carry these microchromosomes in some lineages while others do not, and how they evolve remain open questions. New studies have shown that microchromosomes exhibit certain unique characteristics of genome structure and organization, such as high gene densities, low heterochromatin levels, and high rates of recombination. Our review focuses on recent concepts to expand current knowledge on the dynamic nature of karyotype evolution in vertebrates, raising important questions regarding the evolutionary origins and ramifications of microchromosomes. We introduce the basic karyotypic features to clarify the size, shape, and morphology of macro- and microchromosomes and report their distribution across different lineages. Finally, we characterize the mechanisms of different evolutionary forces underlying the origin and evolution of microchromosomes.