Insights into the karyotype evolution and speciation of the beetle Euchroma gigantea (Coleoptera: Buprestidae).

Euchroma Dejean, 1833 (Buprestidae: Coleoptera) is a monotypic genus comprising the species Euchroma gigantea, with populations presenting a degree of karyotypic variation/polymorphism rarely found within a single taxonomic (specific) unit, as well as drastically incompatible meiotic configurations in populations from extremes of the species range. To better understand the complex karyotypic evolution of E. gigantea, the karyotypes of specimens from five populations in Brazil were investigated using molecular cytogenetics and phylogenetic approaches. Herein, we used FISH with histone genes as well as sequencing of the COI to determine differential distribution of markers and relationships among populations. The analyses revealed new karyotypes, with variability for chromosome number and morphology of multiple sex chromosome mechanisms, occurrence of B chromosome variants (punctiform and large ones), and high dispersion of histone genes in different karyotypes. These data indicate that chromosomal polymorphism in E. gigantea is greater than previously reported, and that the species can be a valuable model for cytogenetic studies. The COI phylogenetic and haplotype analyses highlighted the formation of three groups with chromosomally polymorphic individuals. Finally, we compared the different karyotypes and proposed a model for the chromosomal evolution of this species. The species E. gigantea includes at least three cytogenetically polymorphic lineages. Moreover, in each of these lineages, different chromosomal rearrangements have been fixed. Dispersion of repetitive sequences may have favored the high frequency of these rearrangements, which could be related to both adaptation of the species to different habitats and the speciation process.

Chromosomal diversification of diploid number, heterochromatin and rDNAs in two species of Phanaeus beetles (Scarabaeidae, Scarabaeinae).

The genus Phanaeus is included in the tribe Phanaeini, one of the most diverse tribes within the subfamily Scarabaeinae in terms of chromosomal characteristics. However, so far the species of this genus were not studied with differential cytogenetic techniques, limiting any inference of the probable mechanisms responsible for this diversity. In this work, several techniques were applied with the aim of cytogenetically characterizing two Phanaeus species. The karyotype found for Phanaeus (Notiophanaeus) chalcomelas was 2n = 12, neo-XY, and that of P. (N.) splendidulus was 2n = 20, Xyp, considered primitive for the family Scarabaeidae. The chromosomes of both species showed a high amount of constitutive heterochromatin (CH), with blocks rich in base pairs GC (CMA3 (+)). Moreover, in P. (N.) chalcomelas the marks revealed by C-banding and fluorochrome staining were different in size, showing CH variability. Sites of 18S ribosomal DNA (rDNA) were identified in one autosomal pair of P. (N.) chalcomelas and in five autosomal pairs of P. (N.) splendidulus. On the other hand, only one autosomal pair exhibited 5S rDNA sequences in these species. The results suggest that the karyotype differentiation of the Phanaeus species studied here involved pericentric inversions and centric fusions, as well as mechanisms related to amplification and dispersion of CH and rDNA sequences.

Karyotype characterization of Eurysternus caribaeus: the smallest diploid number among Scarabaeidae (Coleoptera: Scarabaeoidea).

Eurysternus caribaeus was analyzed cytologically by conventional chromosomal staining. The species presents a diploid number of 2n=8, chromosomes with two arms and an XY sex determining mechanism. This is the first karyotype described for the genus Eurysternus and the tribe Eurysternini, and is also the smallest diploid number observed in the family Scarabaeidae and superfamily Scarabaeoidea.

Karyotypic characterization of representatives from Melolonthinae (Coleoptera: Scarabaeidae): karyotypic analysis, banding and fluorescent in situ hybridization (FISH).

Meiotic chromosomes of Phyllophaga (Phytalus) vestita, Phyllophaga (Phyllophaga) aff capillata and Lyogenys fuscus (Melolonthinae) were analyzed by conventional staining, C-banding, fluorochromes, silver nitrate and FISH. The three species had a diploid number of 2n=20 and a sex mechanism of the (Xyp; XYp) parachute type. P. (Phytalus) vestita,P. (Phyllophaga) aff capillata and Lyogenys fuscus showed pericentromeric constitutive heterochromatin (CH) in all autosomal bivalents and on X chromosomes. Staining with CMA3 and DAPI fluorochromes showed that the CH of P. (Phytalus) vestita is not specifically rich in AT and GC-base pairs, whereas in P. (Phyllophaga) aff capillata the sex bivalent and one autosomal pair were found to be enriched in GC base pairs with CMA3, and in Lyogenys fuscus CH was positive for DAPI. Silver nitrate staining revealed nucleolar remnants in all three species. However, FISH obtained a precise identification of nucleolar organizing regions with an rDNA 18S and 25S probe. A signal of hybridization was seen in each species, being detected in the X chromosome of P. (Phytalus) vestita and Lyogenys fuscus, and in a small autosomal bivalent of P. (Phyllophaga) aff capillata.