Nucleolus and chromosome relationships at pachynema in four Scarabaeoidea (Coleoptera) species with various combinations of NOR and sex chromosomes

Multiple Chromosome Fissions, Including That of the X Chromosome, in Aulacocyclus tricuspis Kaup (Coleoptera, Passalidae) from New Caledonia: Characterization of a Rare but Recurrent Pathway of Chromosome Evolution in Animals

The male karyotype of Aulacocyclus tricuspis Kaup 1868 (Coleoptera, Scarabaeoidea, Passalidae, Aulacocyclinae) from New Caledonia contains an exceptionally high number of chromosomes, almost all of which are acrocentric (53,X1X2Y). Unlike the karyotypes of other species of the pantropical family Passalidae, which are principally composed of metacentric chromosomes, this karyotype is derived by fissions involving almost all the autosomes after breakage in their centromere region. This presupposes the duplication of the centromeres. More surprising is the X chromosome fragmentation. The rarity of X chromosome fission during evolution may be explained by the deleterious effects of alterations to the mechanisms of gene dosage compensation (resulting from the over-expression of the unique X chromosome in male insects). Herein, we propose that its occurrence and persistence were facilitated by (1) the presence of amplified heterochromatin in the X chromosome of Passalidae ancestor, and (2) the capacity of heterochromatin to modulate the regulation of gene expression. In A. tricuspis, we suggest that the portion containing the X proper genes and either a gene-free heterochromatin fragment or a fragment containing a few genes insulated from the peculiar regulation of the X by surrounding heterochromatin were separated by fission. Finally, we show that similar karyotypes with multiple acrocentric autosomes and unusual sex chromosomes rarely occur in species of Coleoptera belonging to the families Vesperidae, Tenebrionidae, and Chrysomelidae. Unlike classical Robertsonian evolution by centric fusion, this pathway of chromosome evolution involving the centric fission of autosomes has rarely been documented in animals.

Why Are X Autosome Rearrangements so Frequent in Beetles? A Study of 50 Cases

Amongst the 460 karyotypes of Polyphagan Coleoptera that we studied, 50 (10.8%) were carriers of an X autosome rearrangement. In addition to mitotic metaphase analysis, the correct diagnosis was performed on meiotic cells, principally at the pachytene stage. The percentages of these inter-chromosomal rearrangements, principally fusions, varied in relation to the total diploid number of chromosomes: high (51%) below 19, null at 19, low (2.7%) at 20 (the ancestral and modal number), and slightly increasing from 7.1% to 16.7% from 22 to above 30. The involvement of the X in chromosome fusions appears to be more than seven-fold higher than expected for the average of the autosomes. Examples of karyotypes with X autosome rearrangements are shown, including insertion of the whole X in the autosome (ins(A;X)), which has never been reported before in animals. End-to-end fusions (Robertsonian translocations, terminal rearrangements, and pseudo-dicentrics) are the most frequent types of X autosome rearrangements. As in the 34 species with a 19,X formula, there was no trace of the Y chromosome in the 50 karyotypes with an X autosome rearrangement, which demonstrates the dispensability of this chromosome. In most instances, C-banded heterochromatin was present at the X autosome junction, which suggests that it insulates the gonosome from the autosome portions, whose genes are subjected to different levels of expression. Finally, it is proposed that the very preferential involvement of the X in inter-chromosome rearrangements is explained by: (1) the frequent acrocentric morphology of the X, thus the terminal position of constitutive heterochromatin, which can insulate the attached gonosomal and autosomal components; (2) the dispensability of the Y chromosome, which considerably minimizes the deleterious consequences of the heterozygous status in male meiosis, (3) following the rapid loss of the useless Y chromosome, the correct segregation of the X autosome-autosome trivalent, which ipso facto is ensured by a chiasma in its autosomal portion.

Interstitial NORs, Fragile Sites, and Chromosome Evolution: A Not So Simple Relationship - The Example of Melolontha melolontha and Genus Protaetia (Coleoptera: Scarabaeidae)

In the present study, the origin of recurrent rearrangements involving chromosome 6 in 3.2% of cells of Melolontha melolontha (Coleoptera, Scarabaeidae) was investigated. Various chromosome staining techniques, including C-banding, Giemsa and silver staining, as well as fluorescence in situ hybridization with a human 28S rDNA probe, were applied to M. melolontha chromosome spreads. In addition, related species of the genera Melolontha and Protaetia were studied. On chromosome 6 of M. melolontha, there is a fragile site-like structure which corresponds to an interstitial nucleolus organizer region (NOR). Despite this instability, the NOR remains unique and interstitial in this species, as well as in the other species studied. It is proposed that the intercalary position of the NOR both facilitates the detection of its fragile site-like instability and correlates with its relative stability during evolution. We explain this apparent paradox by strong counter-selection for imbalances of the chromosome fragment distal to the interstitial NORs, which would recurrently occur in the progeny of translocation carriers. Thus, the frequent telomeric position of the NORs in most animal and plant taxa would have no functional rationale but would be the consequence of selection against the meiotic transmission of chromosome imbalances.

Karyotype and sex chromosome differentiation in two Nalassus species (Coleoptera, Tenebrionidae)

Cytogenetic features of Nalassus bozdagus Nabozhenko & Keskin, 2010 and Nalassus plebejus Küster, 1850 were analysed using conventional and differential staining. Mitotic and meiotic chromosomal analysis revealed the diploid number as 2n = 20 (9+Xy_p) in both species. Besides the general resemblance of two Nalassus Mulsant, 1854 karyotypes, important differences related to variations in the number of metacentric/submetacentric chromosomes, localization of highly impregnated regions which are considered as NOR and heterochromatin distribution are clearly observed. The most prominent difference between two species is found related to the X chromosome which is clearly larger in Nalassus bozdagus and has a conspicuous secondary constriction on the long arm. As a result of silver staining, the existence of highly impregnated areas associated with Xy_p of Nalassus bozdagus in both prophase I and metaphase I, suggests that NORs are seemingly located on sex chromosomes. On the other hand, the potential NORs of Nalassus plebejus were observed only in prophase I nuclei. With the application of fluorescence dye DAPI, the AT rich chromosome regions and Xy_p which forms the parachute configuration were shown in both species.