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Abstract
To date, more than 65 species of Brazilian bees (of the superfamily Apoidea) have been cytogenetically studied, but only a few solitary species have been analyzed. One example is the genus Melitoma Lepeletier & Serville, 1828, for which there is no report in the literature with regard to cytogenetic studies. The objective of the present study is to analyze the chromosome number and morphology of the species Melitoma segmentaria (Fabricius, 1804), as well as to determine the pattern of heterochromatin distribution and identify the adenine–thymine (AT)- and guanine–cytosine (GC)-rich regions. Melitoma segmentaria presents chromosome numbers of 2n=30 (females) and n=15 (males). With C-banding, it is possible to classify the chromosomes into seven pseudo-acrocentric pairs (AM), seven pseudo-acrocentric pairs with interstitial heterochromatin (AMi), and one totally heterochromatic metacentric pair (Mh). Fluorochrome staining has revealed that heterochromatin present in the chromosomal arms is rich in GC base pairs (CMA3+) and the centromeric region is rich in AT base pairs (DAPI+). The composition found for Melitoma diverges from the pattern observed in other bees, in which the heterochromatin is usually rich in AT. In bees, few heterochromatic regions are rich in GC and these are usually associated with or localized close to the nucleolus organizer regions (NORs). Silver nitrate impregnation marks the heterochromatin present in the chromosome arms, which makes identification of the NOR in the chromosomes impossible. As this technique reveals proteins in the NOR, the observation that is made in the present study suggests that the proteins found in the heterochromatin are qualitatively similar to those in the NOR.
Highlights
The genus Melitoma Lepeletier & Serville, 1828 belongs to the tribe Emphorini and has 10 described species
The species M. segmentaria showed a chromosome number of 2n=30 for females and n=15 for males (Fig. 1). This chromosome number was similar to that observed in other solitary bee species, including Ceratina megastigmata Yasumatsu and Hirashima, 1969 (2n=34), Xylocopa appendiculata Smith, 1952 (2n=32), and Pithitis smaragdula (Fabricius, 1787) (2n=28) (Hoshiba and Imai 1993)
The C-banding technique allows the observation of large positive heterochromatic blocks in the chromosomes of M. segmentaria (Fig. 1c), wherein, at least one of the arms, has been completely heterochromatic
Summary
The genus Melitoma Lepeletier & Serville, 1828 belongs to the tribe Emphorini and has 10 described species. These are solitary bees, which nest in cavities in the soil, are typically gregarious, and are distributed from Mexico to Argentina (Mamede-Filho et al 1991). These bees are closely associated with a particular plant, Ipomoea sp. Cytogenetic studies of Brazilian bees are more common in the eusocial species belonging to the tribe Meliponini. These studies were initiated by Kerr (1948). The haploid chromosome number in the bees of this tribe ranges from eight to eighteen, where n=17 is the predominant number (Rocha et al 2003)
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