Nucleolar controlled segregation of the sex chromosomes in beetles

An updated Atlas of Helophorus chromosomes

An account is given of my development of techniques to obtain well-spread Giemsa-stained banded chromosome preparations. Apparent G-banding could be obtained following very slight trypsin treatment of freshly prepared slides, but this banding was very fine (close-grained) and possibly not a reflection of chromosome structure. However, treatment of developing embryos in vitro with 5-fluorouridine produced a similar chromomere banding, which is therefore regarded as genuine. Steady accumulation of Helophorus Fabricius, 1775 karyotypes has resulted in the production of an Atlas covering 62 of the 170 species known to occur in the Palaearctic. Chromosome polymorphisms involving pericentric inversions and addition of extra C-banding regions have been found, as well as small B-chromosomes in a few species. In general, karyotypes have proved very useful in establishing the limits of individual species. Parthenogenesis involving triploidy has been found in two species. Karyotypes of experimentally produced hybrids have revealed irregularities in chromosome condensation.

Different behaviour of C-banded peri-centromeric heterochromatin between sex chromosomes and autosomes in Polyphagan beetles

Heterochromatin variation was studied after C-banding of male karyotypes with a XY sex formula from 224 species belonging to most of the main families of Coleoptera. The karyotypes were classified in relation with the ratio heterochromatin/euchromatin total amounts and the amounts of heterochromatin on autosomes and gonosomes were compared. The C-banded karyotypes of 19 species, representing characteristic profiles are presented. This analysis shows that there is a strong tendency for the homogenization of the size of the peri-centromeric C-banded heterochromatin on autosomes. The amount of heterochromatin on the X roughly follows the variations of autosomes. At contrast, the C-banded heterochromatin of the Y, most frequently absent or very small and rarely amplified, looks quite independent from that of other chromosomes. We conclude that the Xs and autosomes, but not the Y, possibly share some, but not all mechanisms of heterochromatin amplification/reduction. The theoretical models of heterochromatin expansion are discussed in the light of these data.

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.

Cytogenetic analysis of Lagria villosa (Coleoptera, Tenebrionidae): emphasis on the mechanism of association of the Xy(p) sex chromosomes

The Xy(p) sex determination mechanism is the system most frequent and ancestral to Coleoptera. Moreover, the presence of argyrophilous material associated with the sex bivalent is described as being responsible for the maintenance and association of these chromosomes. There are no karyotype data available regarding the genus Lagria and no consensus in the literature regarding the argyrophilous material present in the lumen of sex bivalent. Therefore, the aim of this work was to investigate the mechanism of sex chromosome bivalent association in Lagria villosa by analyzing the argyrophilous nature of the material present in the Xy(p) lumen. It was also intended to characterize L. villosa cytogenetically. The analysis of meiotic cells showed 2n = 18 = 16+Xy(p) for males and 2n = 18 = 16+XX in females and the meiotic formula was 2n = 8(II)+Xy(p). The C-banding showed blocks of pericentromeric heterochromatin in all chromosomes except in the y(p) chromosome. In these regions, the use of fluorochromes revealed the presence of heterochromatin containing GC rich DNA sequences. The study of synaptonemal complex showed a gradual increase in the electron-density of the axial elements of the sex chromosomes and their association with strongly electron-dense material. The pepsin pretreatment revealed that the material impregnated by silver is protein.

Strategies of karyotype differentiation in Elateridae (Coleoptera, Polyphaga)

The chromosome study of five species of the family Elateridae, belonging to the subfamilies Agrypninae and Elaterinae, and the analysis of the cytogenetic data previously recorded for this family permitted the establishment of the main strategies of karyotypic differentiation that has occurred in the elaterids. In Agrypninae, the three species studied (Conoderus fuscofasciatus, Conoderus rufidens, and Conoderus sp.) showed the male karyotype 2n=16+X0. This karyotypic uniformity detected in these Conoderus species has also been shared with other species of the same genus, differing considerably from chromosomal heterogeneity verified in the subfamily Agrypninae. The use of the C-banding technique in C. fuscofasciatus and Conoderus sp. revealed constitutive heterochromatin in the pericentromeric region of the majority of the chromosomes. In C. fuscofasciatus, additional constitutive heterochromatin were also observed in the long arm terminal region of almost all chromosomes. Among the representatives of Elaterinae, the karyotype 2n=18+Xy(p) of Pomachilius sp.2 was similar to that verified in the majority of the Coleoptera species, contrasting with the chromosomal formula 2n=18+X0 detected in Cardiorhinus rufilateris, which is most common in the species of Elaterinae. In the majority of the elaterids, the chromosomal differentiation has frequently been driven by reduction of the diploid number; but, among the four cytogenetically examined subfamilies, there are some differences in relation to the trends of karyotypic evolution.

Sequential FISH analysis with rDNA genes and Ag-NOR banding in the lady beetle Olla v-nigrum (Coleoptera: Coccinellidae)

We have characterized the meiosis of Olla v-nigrum by standard analysis, performed a NOR study using NOR banding, FISH of rDNA genes and sequential FISH/AgNOR analysis, and adapted the FISH methodology to Coccinellidae. The chromosome number determined at metaphase I was n = 9 + Xyp. At zygotene it was possible to identify the sex vesicle which presented a deeply stained heteropycnotic block. Chromosome X is much larger than the y and the two combine, forming a "parachute" in metaphase I. FISH analysis using a probe of rDNA genes 18S, 28S and 5.8S of D. melanogaster was used to map the genes in the sex vesicle. The NOR band showed high gene activity in this region. These results were confirmed using sequential FISH/Ag NOR analysis. The data obtained for Olla v-nigrum agree with the classical hypothesis raised to explain the type of sex chromosome association in a parachute format (Xyp) as being due to the presence of nucleolar material. The chromosome number and parachute configuration during metaphase I in this species agree with the basic karyotype of most Coleopterans. The major adaptation of the FISH method was the simultaneous denaturation and hybridization that permitted preservation of chromosome morphology, an essential factor when the chromosomes are small.

Cytogenetic analysis of Epicauta atomaria (Meloidae) and Palembus dermestoides (Tenebrionidae) with Xyp sex determination system using standard staining, C-bands, NOR and synaptonemal complex microspreading techniques

The mitotic and meiotic chromosomes of the beetles Epicauta atomaria (Meloidae) and Palembus dermestoides (Tenebrionidae) were analysed using standard staining, C-banding and silver impregnation techniques. We determine the diploid and haploid chromosome numbers, the sex determination system and describe the chromosomal morphology, the C-banding pattern and the chromosome(s) bearing NORs (nucleolar organizer regions). Both species shown 2n = 20 chromosomes, the chromosomal meioformula 9 + Xyp, and regular chromosome segregation during anaphases I and II. The chromosomes of E. atomaria are basically metacentric or submetacentric and P. dermestoides chromosomes are submetacentric or subtelocentric. In both beetles the constitutive heterochromatin is located in the pericentromeric region in all autosomes and in the Xp chromosome; additional C-bands were observed in telomeric region of the short arm in some autosomes in P. dermestoides. The yp chromosome did not show typical C-bands in these species. As for the synaptonemal complex, the nucleolar material is associated to the 7th bivalent in E. atomaria and 3rd and 7th bivalents in P. dermestoides. Strong silver impregnated material was observed in association with Xyp in light and electron microscopy preparations in these species and this material was interpreted to be related to nucleolar material.

Localization of tandemly repeated DNA sequences in beetle chromosomes by fluorescent in situ hybridization

In situ hybridization to chromosomes and nuclei of Tenebrio molitor shows the massive presence of a species-specific satellite DNA in all chromosomes and six sites of rDNA in mitotic chromosomes. These sites are located in two autosomal pairs and in the X and Y chromosomes. In a related species, Misolampus goudoti, in which two different families of highly repetitive DNA have been previously characterized, one family is located in centromeric regions of all chromosomes with the exception of chromosome Y, while the other repeated DNA family is present both in centromeric and distal regions of all chromosomes. rRNA genes in this species are present in a medium-sized autosomal pair only. These results show that molecular cytogenetics can be applied to coleopteran chromosomes and open the way for a physical mapping of DNA sequences in these organisms. The results also provide insights into the type of meiotic association of the X and Y chromosomes in Coleoptera and the distribution of repeated DNAs within the genome of these insects.

Myths and mechanisms of meiosis

A comparative analysis of the meiotic secquence in a wide variety of organisms indicates there is no convincing evidence that: (1) Premeiotic pairing plays any role in the synapsis of homologues. (2) Heterochromatic association facilitates homologous pairing. (3) Chiasmata ever form within segments which are positively heteropycnotic at zygotenepachytene. (4) Localisation of chiasmata depends on prior localisation of pairing or on the occurrence of euchromatin-heterochromatin boundaries. (5) Prior association of centromeres plays any role in determing co-orientation. (6) Any form of supra-chromosomal organisation exists involving permanent association between the members of a haploid complement, and (7) Unequal progeny ratios recovered from structurally modified Drosophila complements arise as a consequence of distributive pairing.--On the other hand there is good evidence that: (1) Interlocking of bivalents can occur regularly in species with a chiasma frequency sufficiently high to regularly produce ring bivalents and in which the chiasmata are localised to the ends of the bivalent. (2) Some forms of terminal association cannot represent terminalised chiasmata. (3) U-type exchanges present at diplotene result from errors in crossing over. (4) Pairing and chiasma formation are not necessary for coorientation, and (5) at least some types of elastic constrictions present at first metaphase represent extended nucleolar organisers.

STOLAINE CASSIDINES (COLEOPTERA, CHRYSOMELIDAE) WITH Xyp SEX CHROMOSOMES AND A DERIVATIVE SYSTEM XpneoXneoYp

Three species of Cassidines were cytologically studied. Chelymorpha indigesta and C. variabilis, with 10II + Xyp, have a relatively primitive Coleopteran karyotype and the sex chromosomes form a parachute-like bivalent. Botanochara angulata, belonging to the same tribe, Stolaini, presents an exceptional karyotype with a very high chromosome number (2n = 51 ♀) and multiple sex chromosomes XpneoXneoYp. This system has derived from the Xyp by a yp-to-autosome translocation.

CHROMOSOMAL POLYMORPHISM AND INTER-RELATIONSHIPS IN PISSODES WEEVILS: ADDITIONAL CYTOGENETIC EVIDENCE OF SYNONYMY

Chromosome comparison and crossability tests compel relegating 6 of 10 Hopkins' Pissodes taxa to synonymy, namely, utahensis with similis Hopk., nigrae and alascensis with rotundatus Lec., fraseri and piperi with dubius Rand., and curriei with affinis Rand. Two, dubius and rotundatus, and possibly a third, affinis, of the four valid species revealed meiotic markers of semi-incompatibility not previously encountered; presumably, as in the strobi complex, the fourth, similis, is merely devoid of the necessary chromosomal diagnostic.

Low chromosome number and post-reductional XO in Gelus californicus (LEC.) (Coleoptera: Curculionidae)

Gelus californicus is a unique example, among Curculionoidea, of evolutionary mutation to a post-reductional sex-determining mechanism, autosomal behavior having remained perfectly conventional. It has the lowest reported chromosome number in the superfamily, 2n = 12 + XO, and is the only member known with certainty to lack a y. Ovarial preparations were unsuccessful, but of 11 males examined, 9 carried a minute chromosome, presumably a B, a 10th having two, and the 11th, two or four B's in different nuclei. B's are also post-reductional. The X and B only occasionally associate physically at metaphase I, but are more narrowly distributed than expected of non-homologous markers. This spatial association persists until early anaphase II, which finds both elements lagging on the equator. Persistent ectopic association apparently distorts segregation ratios, as the X and B were at the same pole in 17 of 21 late A II plates examined. The A II behavior of these elements effectively rules out an X + y or XO xo ♀ system. The origin of the mechanism and its relationship to other coleopteran examples are discussed.

Multiple sex chromosomes and configuration polymorphism in the Monochamus scutellatus-oregonensis complex (Coleoptera: Cerambycidae)

Fission-fusion polymorphism of the X chromosome and multiple sex chromosome configurations were observed in the Monochamus scutellatus-oregonensis complex. Segregation of fission X chromosomes opposite the Y is ensured by nucleolar orientation. However, occasional XY gametes may result when one X becomes disjoined from the nucleolus before the first meiotic division.

CHROMOSOMAL POLYMORPHISM IN NORTH AMERICAN PISSODES WEEVILS: STRUCTURAL ISOMERISM

Pissodes approximatus and P. canadensis, two taxa named by Hopkins, are shown to constitute a hybrid complex that forms a numerical-morphological karyocline between the southeast of North America and the Yukon. The two A metacentrics and the two B submetacentrics in the SE (predominantly 2n = 30) are each sequentially replaced by twin acrocentrics, aa and bb, in the NW (mainly 2n = 34). The two b pairs, of markedly different length, are segmentally interchanged in the SE relative to those in the NW, the distal interchanged segments being equal. The two Bs, derived by fusion of alternative acrocentrics, simulate pericentrically inverted, complementary submetacentrics. Introgression of the four structurally different types, commonest at the centre of the karyocline, results in 10 distinct B: bb karyotypes of which only 6 appear different; the other 4 are mirror images, or isomers.The intermediate karyotypes have been synthesized by crossing different AB and aabb taxa; they implicate two pairs of isomeric species in the origin of the polymorphic complex. Of these, one AB species with 'standard' Bs and two aabb species with relatively interchanged bs are extant. The fourth, a species with inverted Bs, may be extinct, have merely escaped detection, or else is completely introgressed.Chiasma formation is near perfect in natural polyisomorphs and almost equally so in synthetics. In fact, in both bb/bb heterozygotes the maximum number of associations per cell, normally 16 (the XY pair is achiasmate), is increased by the formation of anomalous quadruple configurations resulting from extra unions at the centric ends of the bs. If chiasmate and situated in minute arms, these unions far exceed expectation on a length-frequency basis, for such arms cannot be resolved in colchicined gonial metaphases. Whether the extra unions are truly chiasmate, as believed, or not, they nevertheless greatly influence the first metaphase disjunctional arrangement of the multiples and cause production of about 80% unbalanced gametes vs. the 50% expected of random assortment, thus generating a genetic isolating mechanism. Such acrocentric heterozygotes, though carrying a heavy segregational-sterility load, persist by recurrent SE-NW introgressive hybridization.