The structure of condensed chromosomes in mitosis and meiosis of insects

H3K9me2 genome-wide distribution in the holocentric insect Spodoptera frugiperda (Lepidoptera: Noctuidae)

BACKGROUND

Eukaryotic genomes are packaged by Histone proteins in a structure called chromatin. There are different chromatin types. Euchromatin is typically associated with decondensed, transcriptionally active regions and heterochromatin to more condensed regions of the chromosomes. Methylation of Lysine 9 of Histone H3 (H3K9me) is a conserved biochemical marker of heterochromatin. In many organisms, heterochromatin is usually localized at telomeric as well as pericentromeric regions but can also be found at interstitial chromosomal loci. This distribution may vary in different species depending on their general chromosomal organization. Holocentric species such as Spodoptera frugiperda (Lepidoptera: Noctuidae) possess dispersed centromeres instead of a monocentric one and thus no observable pericentromeric compartment. To identify the localization of heterochromatin in such species we performed ChIP-Seq experiments and analyzed the distribution of the heterochromatin marker H3K9me2 in the Sf9 cell line and whole 4th instar larvae (L4) in relation to RNA-Seq data.

RESULTS

In both samples we measured an enrichment of H3K9me2 at the (sub) telomeres, rDNA loci, and satellite DNA sequences, which could represent dispersed centromeric regions. We also observed that density of H3K9me2 is positively correlated with transposable elements and protein-coding genes. But contrary to most model organisms, H3K9me2 density is not correlated with transcriptional repression.

CONCLUSION

This is the first genome-wide ChIP-Seq analysis conducted in S. frugiperda for H3K9me2. Compared to model organisms, this mark is found in expected chromosomal compartments such as rDNA and telomeres. However, it is also localized at numerous dispersed regions, instead of the well described large pericentromeric domains, indicating that H3K9me2 might not represent a classical heterochromatin marker in Lepidoptera. (242 words).

Behaviour, feeding and cytogenetic features of the wingless blood-sucking ectoparasite Cyanolicimex patagonicus (Heteroptera: Cimicidae)

Cyanolicimex (Haematosiphoninae) includes a single species, C. patagonicus, which is found in the largest known colony of its avian host Cyanoliseus patagonus (Psittacidae) located in Patagonia (Argentina). Relationships between Cyanolicimex and other genera of Haematosiphoninae are still unclear because this genus shares some characters with other South American genera and possesses some similarities with Hesperocimex from the Neoarctic region. The aim of the present study was to provide additional data of C. patagonicus so as to better understand its relationships with other South American species. We examined some biological features of C. patagonicus in the field and we performed a cytogenetic analysis. We observed in the field that C. patagonicus does not live inside the hollow nests of Cyanoliseus patagonus. The cytogenetic analysis showed that the male karyotype is 2n= 31= 28A+X1X2Y and revealed an achiasmate male meiosis and of the collochore type. Our results together with available cytogenetic data in other cimicids, allow proposing the possible chromosomal rearrangements involved in the chromosomal evolution of C. patagonicus and also contribute to better understand the evolutionary divergence at the chromosomal level within Haematosiphoninae. Based on the whole evidence, we propose to place in four groups the species of Haematosiphoninae cytogenetically hitherto studied.

Advances and Challenges of Using the Sterile Insect Technique for the Management of Pest Lepidoptera

Over the past 30 years, the sterile insect technique (SIT) has become a regular component of area-wide integrated pest management (AW-IPM) programs against several major agricultural pests and vectors of severe diseases. The SIT-based programs have been especially successful against dipteran pests. However, the SIT applicability for controlling lepidopteran pests has been challenging, mainly due to their high resistance to the ionizing radiation that is used to induce sterility. Nevertheless, the results of extensive research and currently operating SIT programs show that most problems with the implementation of SIT against pest Lepidoptera have been successfully resolved. Here, we summarize the cytogenetic peculiarities of Lepidoptera that should be considered in the development and application of SIT for a particular pest species. We also discuss the high resistance of Lepidoptera to ionizing radiation, and present the principle of derived technology based on inherited sterility (IS). Furthermore, we present successful SIT/IS applications against five major lepidopteran pests, and summarize the results of research on the quality control of reared and released insects, which is of great importance for their field performance. In the light of new research findings, we also discuss options for the development of genetic sexing strains, which is a challenge to further improve the applicability of SIT/IS against selected lepidopteran pests.

Limited intrinsic postzygotic reproductive isolation despite chromosomal rearrangements between closely related sympatric species of small ermine moths (Lepidoptera: Yponomeutidae)

In evolutionarily young species and sympatric host races of phytophagous insects, postzygotic incompatibility is often not yet fully developed, but reduced fitness of hybrids is thought to facilitate further divergence. However, empirical evidence supporting this hypothesis is limited. To assess the role of reduced hybrid fitness, we studied meiosis and fertility in hybrids of two closely related small ermine moths, Yponomeuta padella and Yponomeuta cagnagella, and determined the extent of intrinsic postzygotic reproductive isolation. We found extensive rearrangements between the karyotypes of the two species and irregularities in meiotic chromosome pairing in their hybrids. The fertility of reciprocal F1 and, surprisingly, also of backcrosses with both parental species was not significantly decreased compared with intraspecific offspring. The results indicate that intrinsic postzygotic reproductive isolation between these closely related species is limited. We conclude that the observed chromosomal rearrangements are probably not the result of an accumulation of postzygotic incompatibilities preventing hybridization. Alternative explanations, such as adaptation to new host plants, are discussed.

Novel resistance to Cydia pomonella granulovirus (CpGV) in codling moth shows autosomal and dominant inheritance and confers cross-resistance to different CpGV genome groups

Commercial Cydia pomonella granulovirus (CpGV) products have been successfully applied to control codling moth (CM) in organic and integrated fruit production for more than 30 years. Since 2005, resistance against the widely used isolate CpGV-M has been reported from different countries in Europe. The inheritance of this so-called type I resistance is dominant and linked to the Z chromosome. Recently, a second form (type II) of CpGV resistance in CM was reported from a field population (NRW-WE) in Germany. Type II resistance confers reduced susceptibility not only to CpGV-M but to most known CpGV isolates and it does not follow the previously described Z-linked inheritance of type I resistance. To further analyze type II resistance, two CM strains, termed CpR5M and CpR5S, were generated from parental NRW-WE by repeated mass crosses and selection using the two isolates CpGV-M and CpGV-S, respectively. Both CpR5M and CpR5S were considered to be genetically homogeneous for the presence of the resistance allele(s). By crossing and backcrossing experiments with a susceptible CM strain, followed by resistance testing of the offspring, an autosomal dominant inheritance of resistance was elucidated. In addition, cross-resistance to CpGV-M and CpGV-S was detected in both strains, CpR5M and CpR5S. To test the hypothesis that the autosomal inheritance of type II resistance was caused by a large interchromosomal rearrangement involving the Z chromosome, making type I resistance appear to be autosomal in these strains; fluorescence in situ hybridization with bacterial artificial chromosome probes (BAC-FISH) was used to physically map the Z chromosomes of different CM strains. Conserved synteny of the Z-linked genes in CpR5M and other CM strains rejects this hypothesis and argues for a novel genetic and functional mode of resistance in CM populations with type II resistance.

Comparative cytogenetics of Auchenorrhyncha (Hemiptera, Homoptera): a review

A comprehensive review of cytogenetic features is provided for the large hemipteran suborder Auchenorrhyncha, which currently contains approximately 42,000 valid species. This review is based on the analysis of 819 species, 483 genera, and 31 families representing all presently recognized Auchenorrhyncha superfamilies, e.i. Cicadoidea (cicadas), Cercopoidea (spittle bugs), Membracoidea (leafhoppers and treehoppers), Myerslopioidea (ground-dwelling leafhoppers), and Fulgoroidea (planthoppers). History and present status of chromosome studies are described, as well as the structure of chromosomes, chromosome counts, trends and mechanisms of evolution of karyotypes and sex determining systems, their variation at different taxonomic levels and most characteristic (modal) states, occurrence of parthenogenesis, polyploidy, B-chromosomes and chromosome rearrangements, and methods used for cytogenetic analysis of Auchenorrhyncha.

A step to the gigantic genome of the desert locust: chromosome sizes and repeated DNAs

The desert locust (Schistocerca gregaria) has been used as material for numerous cytogenetic studies. Its genome size is estimated to be 8.55 Gb of DNA comprised in 11 autosomes and the X chromosome. Its X0/XX sex chromosome determinism therefore results in females having 24 chromosomes whereas males have 23. Surprisingly, little is known about the DNA content of this locust's huge chromosomes. Here, we use the Feulgen Image Analysis Densitometry and C-banding techniques to respectively estimate the DNA quantity and heterochromatin content of each chromosome. We also identify three satellite DNAs using both restriction endonucleases and next-generation sequencing. We then use fluorescent in situ hybridization to determine the chromosomal location of these satellite DNAs as well as that of six tandem repeat DNA gene families. The combination of the results obtained in this work allows distinguishing between the different chromosomes not only by size, but also by the kind of repetitive DNAs that they contain. The recent publication of the draft genome of the migratory locust (Locusta migratoria), the largest animal genome hitherto sequenced, invites for sequencing even larger genomes. S. gregaria is a pest that causes high economic losses. It is thus among the primary candidates for genome sequencing. But this species genome is about 50 % larger than that of L. migratoria, and although next-generation sequencing currently allows sequencing large genomes, sequencing it would mean a greater challenge. The chromosome sizes and markers provided here should not only help planning the sequencing project and guide the assembly but would also facilitate assigning assembled linkage groups to actual chromosomes.

The Glanville fritillary genome retains an ancient karyotype and reveals selective chromosomal fusions in Lepidoptera

Previous studies have reported that chromosome synteny in Lepidoptera has been well conserved, yet the number of haploid chromosomes varies widely from 5 to 223. Here we report the genome (393 Mb) of the Glanville fritillary butterfly (Melitaea cinxia; Nymphalidae), a widely recognized model species in metapopulation biology and eco-evolutionary research, which has the putative ancestral karyotype of n=31. Using a phylogenetic analyses of Nymphalidae and of other Lepidoptera, combined with orthologue-level comparisons of chromosomes, we conclude that the ancestral lepidopteran karyotype has been n=31 for at least 140 My. We show that fusion chromosomes have retained the ancestral chromosome segments and very few rearrangements have occurred across the fusion sites. The same, shortest ancestral chromosomes have independently participated in fusion events in species with smaller karyotypes. The short chromosomes have higher rearrangement rate than long ones. These characteristics highlight distinctive features of the evolutionary dynamics of butterflies and moths.

Butterflies and moths (Lepidoptera) vary in chromosome number. Here, the authors sequence the genome of the Glanville fritillary butterfly, Melitaea cinxia, show it has the ancestral lepidopteran karyotype and provide insight into how chromosomal fusions have shaped karyotype evolution in butterflies and moths.

In the shadow of phylogenetic uncertainty: the recent diversification of Lysandra butterflies through chromosomal change

The phylogeny of the butterfly genus Lysandra (Lycaenidae, Polyommatinae) has been intractable using both molecular and morphological characters, which could be a result of speciation due to karyotype instability. Here we reconstruct the phylogeny of the group using multi-locus coalescent-based methods on seven independent genetic markers. While the genus is ca. 4.9 Mya old, the diversification of the extant lineages was extremely recent (ca. 1.5 Mya) and involved multiple chromosomal rearrangements. We find that relationships are uncertain due to both incomplete lineage sorting and hybridization. Minimizing the impact of reticulation in inferring the species tree by testing for mitochondrial introgression events yields a partially resolved tree with three main supported clades: L. punctifera+L. bellargus, the corydonius taxa, and L. coridon+the Iberian taxa, plus three independent lineages without apparently close relatives (L. ossmar, L. syriaca and L. dezina). Based on these results and new karyotypic data, we propose a rearrangement recognizing ten species within the genus. Finally, we hypothesize that chromosomal instability may have played a crucial role in the Lysandra recent diversification. New chromosome rearrangements might be fixed in populations after severe bottlenecks, which at the same time might promote rapid sorting of neutral molecular markers. We argue that population bottlenecks might be a prerequisite for chromosomal speciation in this group.

FISH identification of Helicoverpa armigera and Mamestra brassicae chromosomes by BAC and fosmid probes

Since the Bombyx mori genome sequence was published, conserved synteny between B. mori and some other lepidopteran species has been revealed by either FISH (fluorescence in situ hybridization) with BAC (bacterial artificial chromosome) probes or linkage analysis. However, no species belonging to the Noctuidae, the largest lepidopteran family which includes serious polyphagous pests, has been analyzed so far with respect to genome-wide conserved synteny and gene order. For that purpose, we selected the noctuid species Helicoverpa armigera and Mamestra brassicae, both with n = 31 chromosomes. Gene-defined fosmid clones from M. brassicae and BAC clones from a closely related species of H. armigera, Heliothis virescens, were used for a FISH analysis on pachytene chromosomes. We recognized all H. armigera chromosomes from specific cross-hybridization signals of 146 BAC probes. With 100 fosmid clones we identified and characterized all 31 bivalents of M. brassicae. Synteny and gene order were well conserved between the two noctuid species. The comparison with the model species B. mori (n = 28) showed the same phenomenon for 25 of the 28 chromosomes. Three chromosomes (#11, #23 and #24) had two counterparts each in H. armigera and M. brassicae. Since n = 31 is the modal chromosome number in Lepidoptera, the noctuid chromosomes probably represent an ancestral genome organization of Lepidoptera. This is the first identification of a full karyotype in Lepidoptera by means of BAC cross-hybridization between species. The technique shows the potential to expand the range of analyzed species efficiently.

Chromosomal evolution in tortricid moths: conserved karyotypes with diverged features

Moths of the family Tortricidae constitute one of the major microlepidopteran groups in terms of species richness and economic importance. Yet, despite their overall significance, our knowledge of their genome organization is very limited. In order to understand karyotype evolution in the family Tortricidae, we performed detailed cytogenetic analysis of Grapholita molesta, G. funebrana, Lobesia botrana, and Eupoecilia ambiguella, representatives of two main tortricid subfamilies, Olethreutinae and Tortricinae. Besides standard cytogenetic methods, we used fluorescence in situ hybridization for mapping of major rRNA and histone gene clusters and comparative genomic hybridization to determine the level of molecular differentiation of the W and Z sex chromosomes. Our results in combination with available data in the codling moth, Cydia pomonella, and other tortricids allow us a comprehensive reconstruction of chromosomal evolution across the family Tortricidae. The emerging picture is that the karyotype of a common ancestor of Tortricinae and Olethreutinae differentiated from the ancestral lepidopteran chromosome print of n = 31 by a sex chromosome-autosome fusion. This rearrangement resulted in a large neo-sex chromosome pair and a karyotype with n = 30 conserved in most Tortricinae species, which was further reduced to n = 28 observed in Olethreutinae. Comparison of the tortricid neo-W chromosomes showed differences in their structure and composition presumably reflecting stochasticity of molecular degeneration of the autosomal part of the neo-W chromosome. Our analysis also revealed conservative pattern of the histone distribution, which is in contrast with high rDNA mobility. Despite the dynamic evolution of rDNA, we can infer a single NOR-chromosome pair as an ancestral state not only in tortricids but probably in all Lepidoptera. The results greatly expand our knowledge of the genome architecture in tortricids, but also contribute to the understanding of chromosomal evolution in Lepidoptera in general.

Mediterranean species of the spittlebug genus Philaenus: modes of chromosome evolution

The evolution of karyotypes and sex determination system of Philaenus Stål (Auchenorrhyncha: Aphrophoridae) species is studied here in detail. The most plausible scenario of chromosomal rearrangements accompanying phylogenetic differentiation in Philaenus is advanced. It is postulated that the ancestral karyotype of Philaenus was 2n = 24 + X0. Karyotype changes occurred several times independently in the genus. The karyotype of 2n = 22 + X0 (P. spumarius and P. tesselatus) originated from 2n = 24 + X0 by fusion between two autosomal pairs. The neo-XY system (P. arslani, P. loukasi, P. signatus, P. maghresignus, and P. tarifa) also originated from the 24 + X0 karyotype by means of independent fusions between autosomes and the original X chromosome. The neo-X(1)X(2)Y system (P. italosignus) evolved from the 2n = 22 + neo-XY karyotype by an additional fusion between the Y chromosome and one more autosomal pair. The neo-X(n)Y system of P. italosignus is the first reported case of an evolutionarily fixed multiple sex chromosome system in Auchenorrhyncha.

Cytogenetics of the true bug infraorder Cimicomorpha (Hemiptera, Heteroptera): a review

The Cimicomorpha is one of the largest and highly diversified infraorders of the Heteroptera. This group is also highly diversified cytogenetically and demonstrates a number of unusual cytogenetic characters such as holokinetic chromosomes; m-chromosomes; multiple sex chromosome systems; post-reduction of sex chromosomes in meiosis; variation in the presence/absence of chiasmata in spermatogenesis; different types of achiasmate meiosis. We present here a review of essential cytogenetic characters of the Cimicomorpha and outline the chief objectives and goals of future investigations in the field.

Samia cynthia versus Bombyx mori: comparative gene mapping between a species with a low-number karyotype and the model species of Lepidoptera

We performed gene-based comparative FISH mapping between a wild silkmoth, Samia cynthia ssp. with a low number of chromosomes (2n=25-28) and the model species, Bombyx mori (2n=56), in order to identify the genomic components that make up the chromosomes in a low-number karyotype. Mapping of 64 fosmid probes containing orthologs of B. mori genes revealed that the homologues of either two or four B. mori chromosomes constitute the S. c. ricini (Vietnam population, 2n=27♀/28♂, Z0/ZZ) autosomes. Where tested, even the gene order was conserved between S. c. ricini and B. mori. This was also true for the originally autosomal parts of the neo-sex chromosomes in S. c. walkeri (Sapporo population, 2n=26♀/26♂, neo-Wneo-Z/neo-Zneo-Z) and S. cynthia subsp. indet. (Nagano population, 2n=25♀/26♂, neo-WZ₁Z₂/Z₁Z₁Z₂Z₂). The results are evidence for an internal stability of lepidopteran chromosomes even when all autosomes had undergone fusion processes to form a low-number karyotype.

Unprecedented within-species chromosome number cline in the Wood White butterfly Leptidea sinapis and its significance for karyotype evolution and speciation

Background

Species generally have a fixed number of chromosomes in the cell nuclei while between-species differences are common and often pronounced. These differences could have evolved through multiple speciation events, each involving the fixation of a single chromosomal rearrangement. Alternatively, marked changes in the karyotype may be the consequence of within-species accumulation of multiple chromosomal fissions/fusions, resulting in highly polymorphic systems with the subsequent extinction of intermediate karyomorphs. Although this mechanism of chromosome number evolution is possible in theory, it has not been well documented.

Results

We present the discovery of exceptional intraspecific variability in the karyotype of the widespread Eurasian butterfly Leptidea sinapis. We show that within this species the diploid chromosome number gradually decreases from 2n = 106 in Spain to 2n = 56 in eastern Kazakhstan, resulting in a 6000 km-wide cline that originated recently (8,500 to 31,000 years ago). Remarkably, intrapopulational chromosome number polymorphism exists, the chromosome number range overlaps between some populations separated by hundreds of kilometers, and chromosomal heterozygotes are abundant. We demonstrate that this karyotypic variability is intraspecific because in L. sinapis a broad geographical distribution is coupled with a homogenous morphological and genetic structure.

Conclusions

The discovered system represents the first clearly documented case of explosive chromosome number evolution through intraspecific and intrapopulation accumulation of multiple chromosomal changes. Leptidea sinapis may be used as a model system for studying speciation by means of chromosomally-based suppressed recombination mechanisms, as well as clinal speciation, a process that is theoretically possible but difficult to document. The discovered cline seems to represent a narrow time-window of the very first steps of species formation linked to multiple chromosomal changes that have occurred explosively. This case offers a rare opportunity to study this process before drift, dispersal, selection, extinction and speciation erase the traces of microevolutionary events and just leave the final picture of a pronounced interspecific chromosomal difference.

Identification and molecular characterization of a sex chromosome rearrangement causing a soft and pliable (spli) larval body phenotype in the silkworm, Bombyx mori

We carried out genetic and cytogenetic analyses of X-ray-induced deleterious Z chromosomes that result in a soft and pliable (spli) phenotype in the silkworm, Bombyx mori. In a B. mori strain with a spli phenotype, we found the Z chromosome broken between the sch (1-21.5) and od (1-49.6) loci. We also found a chromosomal fragment bearing a fifth-chromosome locus for egg and eye pigmentation fused to a Z chromosome fragment. By means of fluorescence in situ hybridization using bacterial artificial chromosome clones as probes, we confirmed that the fused chromosome is composed of a fragment of chromosome 5 and a fragment of the Z chromosome. Moreover, a predicted gene, GA002017, the Bombyx ortholog of the Drosophila gene acj6 (Bmacj6), was completely deleted by the Z chromosome breakage event. The relationship between Bmacj6 and the spli phenotype is discussed.

Karyotype conservation in 2 populations of the parthenogenetic scorpion Tityus serrulatus (Buthidae): rDNA and its associated heterochromatin are concentrated on only one chromosome

Within the order Scorpiones, the parthenogenetic mode of reproduction has been described for 11 species, 6 of which belong to the genus Tityus. In this work, an investigation of the chromosome characteristics of 2 populations of Tityus serrulatus, the first scorpion species known to be thelytokously parthenogenetic, is described. An analysis of 40 individuals revealed holocentric chromosomes of large, medium, and small sizes and an invariable diploid number of 2n = 12. In addition to the conserved macrokaryotype structure, specific chromosome regions also appeared unchanged within and between the samples studied; that is, each sample displayed only one chromosome carrier of the active nucleolar organizer region containing ribosomal genes (5.8S, 18S, and 28S) and AT-rich heterochromatin associated with the ribosomal DNA. The high conservation of the chromosomal features observed in T. serrulatus differed from that verified in certain species of other groups of animals that possess both holocentric chromosomes and parthenogenetic reproduction. Moreover, the cytogenetic results obtained herein permit us to suggest how the eggs of T. serrulatus develop, whether by apomixis or automixis.

Complex meiotic configuration of the holocentric chromosomes: the intriguing case of the scorpion Tityus bahiensis

Mitotic and meiotic chromosomes of Tityus bahiensis were investigated using light (LM) and transmission electron microscopy (TEM) to determine the chromosomal characteristics and disclose the mechanisms responsible for intraspecific variability in chromosome number and for the presence of complex chromosome association during meiosis. This species is endemic to Brazilian fauna and belongs to the family Buthidae, which is considered phylogenetically basal within the order Scorpiones. In the sample examined, four sympatric and distinct diploid numbers were observed: 2n = 5, 2n = 6, 2n = 9, and 2 = 10. The origin of this remarkable chromosome variability was attributed to chromosome fissions and/or fusions, considering that the decrease in chromosome number was concomitant with the increase in chromosome size and vice versa. The LM and TEM analyses showed the presence of chromosomes without localised centromere, the lack of chiasmata and recombination nodules in male meiosis, and two nucleolar organiser regions carrier chromosomes. Furthermore, male prophase I cells revealed multivalent chromosome associations and/or unsynapsed or distinctly associated chromosome regions (gaps, less-condensed chromatin, or loop-like structure) that were continuous with synapsed chromosome segments. All these data permitted us to suggest that the chromosomal rearrangements of T. bahiensis occurred in a heterozygous state. A combination of various factors, such as correct disjunction and balanced segregation of the chromosomes involved in complex meiotic pairing, system of achiasmate meiosis, holocentric nature of the chromosomes, population structure, and species dispersion patterns, could have contributed to the high level of chromosome rearrangements present in T. bahiensis.

Cytogenetic characterization and AFLP-based genetic linkage mapping for the butterfly Bicyclus anynana, covering all 28 karyotyped chromosomes

Background

The chromosome characteristics of the butterfly Bicyclus anynana, have received little attention, despite the scientific importance of this species. This study presents the characterization of chromosomes in this species by means of cytogenetic analysis and linkage mapping.

Methodology/Principal Findings

Physical genomic features in the butterfly B. anynana were examined by karyotype analysis and construction of a linkage map. Lepidoptera possess a female heterogametic W-Z sex chromosome system. The WZ-bivalent in pachytene oocytes of B. anynana consists of an abnormally small, heterochromatic W-chromosome with the Z-chromosome wrapped around it. Accordingly, the W-body in interphase nuclei is much smaller than usual in Lepidoptera. This suggests an intermediate stage in the process of secondary loss of the W-chromosome to a ZZ/Z sex determination system. Two nucleoli are present in the pachytene stage associated with an autosome and the WZ-bivalent respectively. Chromosome counts confirmed a haploid number of n = 28. Linkage mapping had to take account of absence of crossing-over in females, and of our use of a full-sib crossing design. We developed a new method to determine and exclude the non-recombinant uninformative female inherited component in offspring. The linkage map was constructed using a novel approach that uses exclusively JOINMAP-software for Lepidoptera linkage mapping. This approach simplifies the mapping procedure, avoids over-estimation of mapping distance and increases the reliability of relative marker positions. A total of 347 AFLP markers, 9 microsatellites and one single-copy nuclear gene covered all 28 chromosomes, with a mapping distance of 1354 cM. Conserved synteny of Tpi on the Z-chromosome in Lepidoptera was confirmed for B. anynana. The results are discussed in relation to other mapping studies in Lepidoptera.

Conclusions/Significance

This study adds to the knowledge of chromosome structure and evolution of an intensively studied organism. On a broader scale it provides an insight in Lepidoptera sex chromosome evolution and it proposes a simpler and more reliable method of linkage mapping than used for Lepidoptera to date.

Codling moth cytogenetics: karyotype, chromosomal location of rDNA, and molecular differentiation of sex chromosomes

We performed a detailed karyotype analysis in the codling moth, Cydia pomonella (L.) (Lepidoptera: Tortricidae), the key pest of pome fruit in the temperate regions of the world. The codling moth karyotype consisted of 2n = 56 chromosomes of a holokinetic type. The chromosomes were classified into 5 groups according to their sizes: extra large (3 pairs), large (3 pairs), medium (15 pairs), small (5 pairs), and dot-like (2 pairs). In pachytene nuclei of both sexes, a curious NOR (nucleolar organizer region) bivalent was observed. It carried 2 nucleoli, each associated with one end of the bivalent. FISH with an 18S ribosomal DNA probe confirmed the presence of 2 clusters of rRNA genes at the opposite ends of the bivalent. In accordance with this finding, 2 homologous NOR chromosomes were identified in mitotic metaphase, each showing hybridization signals at both ends. In highly polyploid somatic nuclei, females showed a large heterochromatin body, the so-called sex chromatin or W chromatin. The heterochromatin body was absent in male nuclei, indicating a WZ/ZZ (female/male) sex chromosome system. In keeping with the sex chromatin status, pachytene oocytes showed a sex chromosome bivalent (WZ) that was easily discernible by its heterochromatic W thread. To study molecular differentiation of the sex chromosomes, we employed genomic in situ hybridization (GISH) and comparative genomic hybridization (CGH). GISH detected the W chromosome by strong binding of the Cy3-labelled, female-derived DNA probe. With CGH, both the Cy3-labelled female-derived probe and Fluor-X labelled male-derived probe evenly bound to the W chromosome. This suggested that the W chromosome is predominantly composed of repetitive DNA sequences occurring scattered in other chromosomes but accumulated in the W chromosome. The demonstrated ways of W chromosome identification will facilitate the development of genetic sexing strains desirable for pest control using the sterile insect technique.

Resolution of sex chromosome constitution by genomic in situ hybridization and fluorescence in situ hybridization with (TTAGG) n telomeric probe in some species of Lepidoptera

We have developed a simple method to resolve the sex chromosome constitution in females of Lepidoptera by using a combination of genomic in situ hybridization (GISH) and fluorescence in situ hybridization with (TTAGG)( n ) telomeric probe (telomere-FISH). In pachytene configurations of sex chromosomes, GISH differentiated W heterochromatin and telomere-FISH detected the chromosome ends. With this method we showed that Antheraea yamamai has a standard system with a fully differentiated W-Z sex chromosome pair. In Orgyia antiqua, we confirmed the presence of neo-W and neo-Z chromosomes, which most probably originated by fusion of the ancestral W and Z with an autosome pair. In contrast to earlier data, Orgyia thyellina females displayed a neo-ZW(1)W(2) sex chromosome constitution. A neo-WZ(1)Z(2) trivalent was found in females of Samia cynthia subsp. indet., originating from a population in Nagano, Japan. Whereas another subspecies collected in Sapporo, Japan, and determined as S. cynthia walkeri, showed a neo-W/neo-Z bivalent similar to O. antiqua, and the subspecies S. cynthia ricini showed a Z univalent (a Z/ZZ system). The combination of GISH and telomere-FISH enabled us to acquire not only reliable information about sex chromosome constitution but also an insight into sex chromosome evolution in Lepidoptera.

Analysis of insect holocentric chromosomes by atomic force microscopy

In order to go in depth into the analysis of holocentric chromosome structure, atomic force microscopy (AFM) was applied to metaphase plates of the aphid Megoura viciae. AFM showed that aphid chromatids adhere to one another without any prominent structure detectable between them and without any evidence of chromosomal constrictions. AFM thus provided new and reliable evidences at a nanomolecular level concerning the holocentric structure of aphid chromosomes, without any of the artefacts due to sample staining or coating that are usually associated with electron microscopy.

Meiotic pairing of sex chromosome fragments and its relation to atypical transmission of a sex-linked marker in Ephestia kuehniella (Insecta: Lepidoptera)

The physical basis of non-Mendelian segregation of a sex-linked marker was studied in sex- chromosome mutant females of eight ASF ('abnormal segregating females') lines in the flour moth, Ephestia kuehniella. Electron microscopical analysis of microspread synaptonemal complexes revealed that in one line, the Z chromosome segment that contained the dz+ allele was translocated onto an autosome. The resulting quadrivalent visible in early female meiosis was 'corrected' into two bivalents in later stages. This explains autosomal inheritance of the sex chromosome marker in this strain. In the other seven ASF lines, the type of meiotic pairing of an additional fragment (Zdz+) of the Z chromosome was responsible for abnormal segregation of the marker gene. In several of these lines, Zdz+ contained a piece of the W chromosome in addition to the Z segment, as was confirmed by comparative genomic hybridization (CGH). Zdz+ formed three alternative pairing configurations with the original sex chromosomes: (i) a WZZdz+ trivalent, (ii) a WZ bivalent and a Zdz+ univalent or (iii) a ZZdz+ bivalent and a W univalent. In the most frequent WZZdz+ configuration, Zdz+ synapsed with Z and, consequently, segregated with W, simulating W linkage. This explains the predominant occurrence of the parental phenotypes in the progeny. Zdz+ univalents or W univalents, on the other hand, segregated randomly, resulting in both parental and nonparental phenotypes. In two of these lines, the Zdz+ was transmitted only to females. The results suggest that the W chromosome segment in Zdz+ of these lines contains a male-killing factor which makes it incompatible with male development. Our data provide direct evidence for the regular transmission of radiation-induced fragments from lepidopteran chromosomes through more than 50 generations. This is facilitated by the holokinetic nature of lepidopteran chromosomes. We conclude that Zdz+ fragments may persist as long as they possess active kinetochore elements.

Chromosomal principle of radiation-induced F1 sterility in Ephestia kuehniella (Lepidoptera: Pyralidae)

A dose-response analysis of chromosomal aberrations was performed in male progeny of gamma-irradiated males in the flour moth, Ephestia kuehniella. For comparison, several female progeny from each dose level were examined. Aberrations were detected on microspread preparations of pachytene nuclei in the electron microscope and classified according to pairing configurations of synaptonemal complexes (SCs). Fragmentation and various translocations were the most numerous aberrations, whereas interstitial deletion and inversion were rare. At 100 Gy, relatively simple multiple translocations were found. Multiple translocations showing complicated configurations occurred at 150 and 200 Gy, and their number increased with the dose. In males, the mean number of chromosomal breaks resulting in aberrations linearly increased with the dose from 8.4 to 16.2 per nucleus. In females, this value achieved a maximum of 11.2 breaks/nucleus at 200 Gy. Three factors were suggested to contribute to the reported higher level of F1 sterility in males than females: (i) survival of males with high numbers of breaks, (ii) crossing-over in spermatogenesis but not in the achiasmatic oogenesis, and (iii) a higher impact of induced changes on the fertility of males than females. It was concluded that translocations are most responsible for the production of unbalanced gametes resulting in sterility of F1 moths. However, F1 sterility predicted according to the observed frequency of aberrations was much higher than the actual sterility reported earlier. This suggests a regulation factor which corrects the predicted unbalanced state towards balanced segregation of translocated chromosomes.

Fine structure of the kinetochores in six species of the Coleoptera

Kinetochore structure was examined in a total of 6 species from 5 different families of the Coleoptera using transmission electron microscopy of ultrathin serial sections. Metaphase spermatogonia and primary and secondary spermatocytes were studied in Tenebrio molitor (Tenebrionidae) to determine whether kinetochore structure varies depending on the cell type. In all three cell types, the kinetochore microtubules (MTs) were in direct contact with the chromosomal surface, and kinetochore plates were not detectable. In the other species, only metaphase I spermatocytes were examined. As in T. molitor, distinct kinetochore plates were also absent in Adelocera murina (Elateridae), Agapanthia villosoviridescens (Cerambycidae), and Coccinella septempunctata (Coccinellidae). However, bivalents in male meiosis of two representatives of the Chrysomelidae, Agelastica alni and Chrysolina graminis, showed roughly spherical kinetochores at their poleward surfaces. Microtubules were in contact with this material. Thus, although the present survey covers only a small number of species, it is clear that at least two kinetochore types occur in the Coleoptera. The cytological findings are discussed in the context of chromosome number and genome size variability in the Coleopteran families studied. It is suggested that properties of the kinetochores could play a role in karyotype evolution in the Coleoptera.