Karyological evidence for interspecific hybridization between Cichla monoculus and C. temensis (Perciformes, Cichlidae) in the Amazon

摘要

In the mid 1950s the peacock bass or tucunaré (Cichla spp.) became very popular and were introduced in several lakes and culture ponds throughout Brazil, as well as in other countries (Nascimento et al. 2001). The genus Cichla, a widely distributed endemic Neotropical cichlid in the Amazon basin, consists of five species which have been described in the literature: C. temensis, restricted to clear and black water tributaries such as the Orinoco, Negro and Tapajós Rivers; C. monoculus, distributed throughout central Amazonia; C. orinocencis, in the Orinoco and Negro basins; C. intermedia, found in the upper Negro and middle Orinoco Rivers; and C. ocellaris, found in Suriname (Machado-Allison 1971; Kullander 1986; Kullander and Nijssen 1989). However, it has been indicated that there are at least seven more species to be described (S. O. Kullander, pers. com.).Despite the importance of the Cichla species, both as a food resource for the general population, and sport fishing for tourists, since late 1970s (Thompson 1979), no cytogenetic study has been published on the genus, although such studies have been presented in meetings and two (unpublished) master's theses (Alves 1998; Nishiyama 1999) have been generated. According to Thompson (1979)Cichla temensis have 48 acrocentric chromosomes.Recently, a presumed new species of Cichla from the Uatum? River, that together with C. monoculus and C. temensis occurs in Lake Balbina (a large manmade reservoir created in the middle of the Brazilian Amazon to supply hydroelectric power) was reported. Lake Balbina (formed in 1987) and its flooded water surface area of 2400 km2, mostly of forest and inundated trunks of dead and leafless trees, recently provided an appropriate habitat for the explosive reproduction of peacock bass. In Lake Balbina, C. monoculus is the most common peacock bass and Cichla sp. the least common. Cichla sp. presents intermediate morphometric characteristics between C. monoculus and C. temensis (Oliveira Júnior 1998).The radically disturbed habitat that must have prevailed during the formation of Lake Balbina as well as the morphometric similarities between the peacock bass species taken from Lake Balbina suggest that Cichla sp. (sensu Oliveira Júnior 1998) is a natural hybrid. Taking this into account, Andrade et al. (2001) analyzed the 16S mtDNA gene of C. monoculus, C. temensis and the presumed Cichla hybrid and confirmed that natural hybridization occurs between C. monoculus and C. temensis in Lake Balbina. They also showed that hybridization events are quite common in many localities in the Amazon, where the two species occur sympatrically.The incidence of hybridization of fish species is not rare and there is evidence of hybridization being a mechanism for speciation. Thus, hybridization may be viewed as another source of variation in evolution where hybrids represent novelties and the new allele combinations could promote adaptive radiation (Arnold 1997). Anthropogenic disturbances, related to aquacultural activities, species introductions and loss or alteration of habitats, are the major contributing factors in interspecific hybridization of fish species (Scribner et al. 2001).As part of an ongoing effort to karyotype the Amazonian cichlids, the aim of this article is to analyze the chromosomes of C. monoculus and C. temensis from the central Amazon, and addressing a particular problem involving natural hybridization between them in Lake Balbina.