Highly Rearranged Karyotypes and Multiple Sex Chromosome Systems in Armored Catfishes from the Genus Harttia (Teleostei, Siluriformes)

Abstract

Harttia comprises an armored catfish genus endemic to the Neotropical region, including 27 valid species with low dispersion rates that are restricted to small distribution areas. Cytogenetics data point to a wide chromosomal diversity in this genus due to changes that occurred in isolated populations, with chromosomal fusions and fissions explaining the 2n number variation. In addition, different multiple sex chromosome systems and rDNA loci location are also found in some species. However, several Harttia species and populations remain to be investigated. In this study, Harttia intermontana and two still undescribed species, morphologically identified as Harttia sp. 1 and Harttia sp. 2, were cytogenetically analyzed. Harttia intermontana has 2n = 52 and 2n = 53 chromosomes, while Harttia sp. 1 has 2n = 56 and 2n = 57 chromosomes in females and males, respectively, thus highlighting the occurrence of an XX/XY1Y2 multiple sex chromosome system in both species. Harttia sp. 2 presents 2n = 62 chromosomes for both females and males, with fission events explaining its karyotype diversification. Chromosomal locations of the rDNA sites were also quite different among species, reinforcing that extensive rearrangements had occurred in their karyotype evolution. Comparative genomic hybridization (CGH) experiments among some Harttia species evidenced a shared content of the XY1Y2 sex chromosomes in three of them, thus pointing towards their common origin. Therefore, the comparative analysis among all Harttia species cytogenetically studied thus far allowed us to provide an evolutionary scenario related to the speciation process of this fish group.

Adding New Pieces to the Puzzle of Karyotype Evolution in Harttia (Siluriformes, Loricariidae): Investigation of Amazonian Species

“…In a previous chromosomal study in Harttia species, 2n = 58 was proposed as the likely ancestral 2n for this genus [ 34 ]. A few years later, the karyotype diversity among Harttia species was described as the result of three different evolutionary pathways: (i) the maintenance of the ancestral 2n = 58 up to today; (ii) the elevation of 2n by centric fissions, and (iii) the 2n reduction by fusions [ 35 ]. Here, we add new pieces to this puzzle by analyzing the karyotypes of another three Harttia species.…”

“…As a final step for all fluorescence assays, chromosomes were counterstained with 4′,6-diamidino-2-phenylindole (DAPI), and the slides were mounted in an antifade solution (VECTASHIELD; Vector Laboratories, Burlingame, CA, USA). Two sets of comparative genomic hybridization (CGH) experiments were designed to seek for the occurrence of sex chromosomes, both following the protocol of Sember et al [ 47 ], with adaptations on probe/C 0 t−1 DNA ratio based on previous Harttia studies [ 35 , 36 ]. In the first set of experiments, male and female genomic probes were co-hybridized to the male chromosomes (i.e., the expected heterogametic sex), in each species separately, to detect regions with specific or biased hybridization of male probe which might point to regions with sex-specific repetitive DNA accumulation on sex chromosomes.…”

“…In this context, the genus Harttia can be recognized as a remarkable model for cytogenetic studies, especially to investigate the evolutionary processes related to sex chromosomes, diversification and karyotype dynamics [ 33 , 34 , 35 , 36 ]. Harttia harbors 27 recognized species [ 1 , 37 ], of which only 14 have been cytogenetically analyzed to date, along with several other Harttia spp.…”

“…Harttia harbors 27 recognized species [ 1 , 37 ], of which only 14 have been cytogenetically analyzed to date, along with several other Harttia spp. waiting for proper taxonomic description (summarized in [ 35 ]). This genus also encompasses the second-largest variation in the 2n among Loricariidae, ranging from 2n = 52 (females)/53 (males) in Harttia carvalhoi [ 33 ] to 2n = 62 in H .…”

“…absaberi and Harttia sp. 2 [ 35 , 38 ]. Furthermore, three different male-heterogametic sex chromosome systems are known or have been suggested to exist among Harttia species: XX/XY 1 Y 2 in H .…”