Estudo citogenético clássico e molecular em quinze espécies da tribo Ancistrini (Siluriformes, Loricariidae) de três bacias hidrográficas brasileiras

Abstract

The Loricariidae Family is the second largest family of fresh water fish with approximately 700 species. Among the Loricariidae, the Ancistrini tribe distinguishes itself with 217 valid species distributed in the neotropical region. The catfish of this group measure an average of 15 centimeters in length and have moveable evertible odontodes in the operculum region. Cytogenetic studies have been made in only 10% of the species. In this study, fifteen species, two of the genus Lasiancistrus and thirteen of the genus Ancistrus were cytogenetically analyzed. Besides the classic analyses of cytogenetic, the technique of fluorescent in situ hybridization (FISH) was used to locate the regions of rDNA 5S and 18S in seven species of the genus Ancistrus. The results showed diploid numbers of 2n=34, 40, 42, 50, 52 and 54 chromosomes. In the species of the genus Lasiacistrus, the diploid number was 2n=54 chromosomes, and there were simple NORs and some small blocks of constitutive heterochromatin. There were no chromosomic differences observed between males and females but there were distinct cytotypes among the species. The results are compared with analyses made with other genera of the Ancistrini group. In the genus Ancistrus diversity of species and chromosomic variation is surprising. In the species Ancistrus cuiabae chromosomic polymorphism and rDNA 18S were observed. The event that is probably responsible for this chromosomic polymorphism was a pericentric inversion. In four species of distinct populations chromosomic heteromorphism and constitutive heterochromatin were verified, thus suggesting that two populations possess XX/XY and ZZ/ZW sexual chromosomes. The nucleolar organizing regions (NORs) that appeared in the presence of silver nitrate were simple in all the species, located in the terminal and interstitial region of the chromosomes. With the FISH technique it was possible to confirm the data obtained with AgNOR in seven species. In respect to rDNA 5S the results were very diversified with only one pair of chromosomes marked, in Ancistrus sp 06; two pairs in A. sp 04, A. sp 08 and A. sp 13; three pairs in A. cuiabae, A. claro and A. cf. dubius. The cístrons observed in rDNA 5S and 18S were syntenic in four of the seven species analyzed. Using technique of C band it was possible to verify the blocks of constitutive heterochromatin were few in number and small in size in the species with diploid numbers 2n=50 to 54, but were bigger and stained in an evident manner in the species with chromosomic polymorphisms, sexual chromosomes and 2n=34 to 42 chromosomes. Besides the highly variable diploid number, the Ancistrini tribe has simple and multiple sexual chromosomes systems. In this study we confirmed the occurrence of chromosomic polymorphism, of rDNA 18S and of constitutive heterochromatin in new species. For the first time fluorescent in situ hybridization was done in the Ancistrini tribe and it confirmed the diversity existent, especially in the genus Ancistrus, which is the most derived of the tribe. With these results it was to infer that the species of the genus Ancistrus with diploid numbers 2n=50 to 54 chromosomes are more primitive than the others founded in the Baixada Cuiabana with intense sedimentation. New analyses, using molecular techniques could help in understanding the phylogeny and evolution of this diversified group of Loricariidae.