Abstract
Four Neotropical tiger beetle species, three from the genus Megacephala and one from the genus Oxycheila, currently assigned to the tribe Megacephalini were examined cytogenetically. All three Megacephala species showed simple sex chromosome systems of the X0/XX type but different numbers of autosomal pairs (15 in M. cruciata, 14 in M. sobrina and 12 in M. rutilans), while Oxycheila tristis was inferred to have a multiple sex chromosome system with four X chromosomes (2n = 24 + X1X2X3X4Y/X1X1X2X2X3X3X4X4). Fluorescence in situ hybridization (FISH) using a PCR-amplified 18S rDNA fragment as a probe revealed the presence of rDNA clusters located exclusively on the autosomes in all the Megacephala species (five clusters in M. cruciata, eight in M. sobrina and three in M. rutilans), indicating variability in the number of clusters and the presence of structural polymorphisms. The same methodology showed that O. tristis had six rDNA clusters, apparently also located on the autosomes. Although our data also show cytogenetic variability within the genus Megacephala, our findings support the most accepted hypothesis for chromosome evolution in the family Cicindelidae. The description of multiple sex chromosomes in O. tristis along with phylogenetic analyses and larval morphological characters may be assumed as an additional evidence for the exclusion of the genus Oxycheila and related taxa from the tribe Megacephalini.
Highlights
The autosomal pairs of the three Megacephala species gradually decreased in size and consisted of metacentrics and submetacentrics (Figures 1, 2 and 3)
The single sex chromosome system (X0) found in M. cruciata, M. rutilans and M. sobrina parallels that found in other Megacephala species, albeit only a few species have so far been studied (Table 2)
We found that Megacephala species have higher and more variable number of autosomal pairs as compared to the Cicindelini with most Megacephala species having from 12 to 15 autosomal pairs, Proença et al (2002b) found an unusually low number of 5 pairs for M. brasiliensis which, was attributed to multiple Robertsonian fusions between the autosomes that might have occurred secondarily in the evolutionary process
Summary
Over the last few years, cytogenetics placed in a phylogenetic context has become a useful tool to provide insights into chromosome evolution within the family Cicindelidae, a Coleopteran group with about 2300 described species distributed worldwide (Cassola and Pearson, 2000; Cassola, 2001). Cicindelidae species belonging to the Collyrini, Ctenostomini and Cicindelini tribes have been described as having a diverse multiple sex chromosome system, XnY, where n varies from two to four (Serrano and Galián, 1998; Galián and Hudson, 1999; Proença et al, 1999a, b, 2002a, 2004; Galián et al, 2002; Zacaro et al, 2004) The number and distribution of these highly repetitive and conserved rDNA clusters can be useful for the construction of physical maps for comparative genomics and phylogenetic and evolutionary studies These studies have revealed the presence of a high number of rDNA loci in representatives of the basal lineages Manticorini, Omini and Megacephalini, such loci being exclusively located on the autosomes (three and four pairs), with lower numbers in the most derived lineages occurring on autosomes, heterosomes or on both. The aim of the work presented in this paper was to describe the karyotypes, sex chromosome determination systems and the localization of major rDNA clusters in four species of Neotropical Megacephalini, which allowed to detect variability within this tribe and within the genus Megacephala
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