Sex-biased gene content is associated with sex chromosome turnover in Danaini butterflies.

Sex chromosomes play an outsized role in adaptation and speciation, and thus deserve particular attention in evolutionary genomics. In particular, fusions between sex chromosomes and autosomes can produce neo-sex chromosomes, which offer important insights into the evolutionary dynamics of sex chromosomes. Here, we investigate the evolutionary origin of the previously reported Danaus neo-sex chromosome within the tribe Danaini. We assembled and annotated genomes of Tirumala septentrionis (subtribe Danaina), Ideopsis similis (Amaurina), Idea leuconoe (Euploeina) and Lycorea halia (Itunina) and identified their Z-linked scaffolds. We found that the Danaus neo-sex chromosome resulting from the fusion between a Z chromosome and an autosome corresponding to the Melitaea cinxia chromosome (McChr) 21 arose in a common ancestor of Danaina, Amaurina and Euploina. We also identified two additional fusions as the W chromosome further fused with the synteny block McChr31 in I. similis and independent fusion occurred between ancestral Z chromosome and McChr12 in L. halia. We further tested a possible role of sexually antagonistic selection in sex chromosome turnover by analysing the genomic distribution of sex-biased genes in I. leuconoe and L. halia. The autosomes corresponding to McChr21 and McChr31 involved in the fusions are significantly enriched in female- and male-biased genes, respectively, which could have hypothetically facilitated fixation of the neo-sex chromosomes. This suggests a role of sexual antagonism in sex chromosome turnover in Lepidoptera. The neo-Z chromosomes of both I. leuconoe and L. halia appear fully compensated in somatic tissues, but the extent of dosage compensation for the ancestral Z varies across tissues and species.

Masculinizer gene controls male sex determination in the codling moth, Cydia pomonella.

The molecular mechanisms of sex determination in moths and butterflies (Lepidoptera) with female heterogamety (WZ/ZZ) are poorly understood, except in the silkworm Bombyx mori. However, the Masculinizer (Masc) gene that controls male development and dosage compensation in B. mori, appears to be conserved in Lepidoptera, as its masculinizing function was recently confirmed in several moth species. In this work, we investigated the role of the Masc gene in sex determination of the codling moth Cydia pomonella (Tortricidae), a globally important pest of pome fruits and walnuts. The gene structure of the C. pomonella Masc ortholog, CpMasc, is similar to B. mori Masc. However, unlike B. mori, we identified 14 splice variants of CpMasc in the available transcriptomes. Subsequent screening for sex specificity and genetic variation using publicly available data and RT-PCR revealed three male-specific splice variants. Then qPCR analysis of these variants revealed sex-biased expression showing a peak only in early male embryos. Knockdown of CpMasc by RNAi during early embryogenesis resulted in a shift from male-to female-specific splicing of the C. pomonella doublesex (Cpdsx) gene, its downstream effector, in ZZ embryos, leading to a strongly female-biased sex ratio. These data clearly demonstrate that CpMasc functions as a masculinizing gene in the sex-determining cascade of C. pomonella. Our study also showed that CpMasc transcripts are provided maternally, as they were detected in unfertilized eggs after oviposition and in mature eggs dissected from virgin females. This finding is unique, as maternal provision of mRNA has rarely been studied in Lepidoptera.

The Role of Repetitive Sequences in Repatterning of Major Ribosomal DNA Clusters in Lepidoptera.

Genes for major ribosomal RNAs (rDNA) are present in multiple copies mainly organized in tandem arrays. The number and position of rDNA loci can change dynamically and their repatterning is presumably driven by other repetitive sequences. We explored a peculiar rDNA organization in several representatives of Lepidoptera with either extremely large or numerous rDNA clusters. We combined molecular cytogenetics with analyses of second- and third-generation sequencing data to show that rDNA spreads as a transcription unit and reveal association between rDNA and various repeats. Furthermore, we performed comparative long read analyses among the species with derived rDNA distribution and moths with a single rDNA locus, which is considered ancestral. Our results suggest that satellite arrays, rather than mobile elements, facilitate homology-mediated spread of rDNA via either integration of extrachromosomal rDNA circles or ectopic recombination. The latter arguably better explains preferential spread of rDNA into terminal regions of lepidopteran chromosomes as efficiency of ectopic recombination depends on the proximity of homologous sequences to telomeres.

Accumulation of retrotransposons contributes to W chromosome differentiation in the willow beauty Peribatodes rhomboidaria (Lepidoptera: Geometridae).

The W chromosome of Lepidoptera is typically gene-poor, repeat-rich and composed of heterochromatin. Pioneering studies investigating this chromosome reported an abundance of mobile elements. However, the actual composition of the W chromosome varies greatly between species, as repeatedly demonstrated by comparative genomic hybridization (CGH) or fluorescence in situ hybridization (FISH). Here we present an analysis of repeats on the W chromosome in the willow beauty, Peribatodes rhomboidaria (Geometridae), a species in which CGH predicted an abundance of W-enriched or W-specific sequences. Indeed, comparative analysis of male and female genomes using RepeatExplorer identified ten putative W chromosome-enriched repeats, most of which are LTR or LINE mobile elements. We analysed the two most abundant: PRW LINE-like and PRW Bel-Pao. The results of FISH mapping and bioinformatic analysis confirmed their enrichment on the W chromosome, supporting the hypothesis that mobile elements are the driving force of W chromosome differentiation in Lepidoptera. As the W chromosome is highly underrepresented in chromosome-level genome assemblies of Lepidoptera, this recently introduced approach, combining bioinformatic comparative genome analysis with molecular cytogenetics, provides an elegant tool for studying this elusive and rapidly evolving part of the genome.

Sex chromosome differentiation via changes in the Y chromosome repeat landscape in African annual killifishes Nothobranchius furzeri and N. kadleci.

Homomorphic sex chromosomes and their turnover are common in teleosts. We investigated the evolution of nascent sex chromosomes in several populations of two sister species of African annual killifishes, Nothobranchius furzeri and N. kadleci, focusing on their under-studied repetitive landscape. We combined bioinformatic analyses of the repeatome with molecular cytogenetic techniques, including comparative genomic hybridization, fluorescence in situ hybridization with satellite sequences, ribosomal RNA genes (rDNA) and bacterial artificial chromosomes (BACs), and immunostaining of SYCP3 and MLH1 proteins to mark lateral elements of synaptonemal complexes and recombination sites, respectively. Both species share the same heteromorphic XY sex chromosome system, which thus evolved prior to their divergence. This was corroborated by sequence analysis of a putative master sex determining (MSD) gene gdf6Y in both species. Based on their divergence, differentiation of the XY sex chromosome pair started approximately 2 million years ago. In all populations, the gdf6Y gene mapped within a region rich in satellite DNA on the Y chromosome long arms. Despite their heteromorphism, X and Y chromosomes mostly pair regularly in meiosis, implying synaptic adjustment. In N. kadleci, Y-linked paracentric inversions like those previously reported in N. furzeri were detected. An inversion involving the MSD gene may suppress occasional recombination in the region, which we otherwise evidenced in the N. furzeri population MZCS-121 of the Limpopo clade lacking this inversion. Y chromosome centromeric repeats were reduced compared with the X chromosome and autosomes, which points to a role of relaxed meiotic drive in shaping the Y chromosome repeat landscape. We speculate that the recombination rate between sex chromosomes was reduced due to heterochiasmy. The observed differences between the repeat accumulations on the X and Y chromosomes probably result from high repeat turnover and may not relate closely to the divergence inferred from earlier SNP analyses.

Paramutation-like Epigenetic Conversion by piRNA at the Telomere of Drosophila virilis.

First discovered in maize, paramutation is a phenomenon in which one allele can trigger an epigenetic conversion of an alternate allele. This conversion causes a genetically heterozygous individual to transmit alleles that are functionally the same, in apparent violation of Mendelian segregation. Studies over the past several decades have revealed a strong connection between mechanisms of genome defense against transposable elements by small RNA and the phenomenon of paramutation. For example, a system of paramutation in Drosophila melanogaster has been shown to be mediated by piRNAs, whose primary function is to silence transposable elements in the germline. In this paper, we characterize a second system of piRNA-mediated paramutation-like behavior at the telomere of Drosophila virilis. In Drosophila, telomeres are maintained by arrays of retrotransposons that are regulated by piRNAs. As a result, the telomere and sub-telomeric regions of the chromosome have unique regulatory and chromatin properties. Previous studies have shown that maternally deposited piRNAs derived from a sub-telomeric piRNA cluster can silence the sub-telomeric center divider gene of Drosophila virilis in trans. In this paper, we show that this silencing can also be maintained in the absence of the original silencing allele in a subsequent generation. The precise mechanism of this paramutation-like behavior may be explained by either the production of retrotransposon piRNAs that differ across strains or structural differences in the telomere. Altogether, these results show that the capacity for piRNAs to mediate paramutation in trans may depend on the local chromatin environment and proximity to the uniquely structured telomere regulated by piRNAs. This system promises to provide significant insights into the mechanisms of paramutation.

Degenerated, Undifferentiated, Rearranged, Lost: High Variability of Sex Chromosomes in Geometridae (Lepidoptera) Identified by Sex Chromatin.

Sex chromatin is a conspicuous body that occurs in polyploid nuclei of most lepidopteran females and consists of numerous copies of the W sex chromosome. It is also a cytogenetic tool used to rapidly assess the W chromosome presence in Lepidoptera. However, certain chromosomal features could disrupt the formation of sex chromatin and lead to the false conclusion that the W chromosome is absent in the respective species. Here we tested the sex chromatin presence in 50 species of Geometridae. In eight selected species with either missing, atypical, or normal sex chromatin patterns, we performed a detailed karyotype analysis by means of comparative genomic hybridization (CGH) and fluorescence in situ hybridization (FISH). The results showed a high diversity of W chromosomes and clarified the reasons for atypical sex chromatin, including the absence or poor differentiation of W, rearrangements leading to the neo-W emergence, possible association with the nucleolus, and the existence of multiple W chromosomes. In two species, we detected intraspecific variability in the sex chromatin status and sex chromosome constitution. We show that the sex chromatin is not a sufficient marker of the W chromosome presence, but it may be an excellent tool to pinpoint species with atypical sex chromosomes.

Large-scale comparative analysis of cytogenetic markers across Lepidoptera.

Fluorescence in situ hybridization (FISH) allows identification of particular chromosomes and their rearrangements. Using FISH with signal enhancement via antibody amplification and enzymatically catalysed reporter deposition, we evaluated applicability of universal cytogenetic markers, namely 18S and 5S rDNA genes, U1 and U2 snRNA genes, and histone H3 genes, in the study of the karyotype evolution in moths and butterflies. Major rDNA underwent rather erratic evolution, which does not always reflect chromosomal changes. In contrast, the hybridization pattern of histone H3 genes was well conserved, reflecting the stable organisation of lepidopteran genomes. Unlike 5S rDNA and U1 and U2 snRNA genes which we failed to detect, except for 5S rDNA in a few representatives of early diverging lepidopteran lineages. To explain the negative FISH results, we used quantitative PCR and Southern hybridization to estimate the copy number and organization of the studied genes in selected species. The results suggested that their detection was hampered by long spacers between the genes and/or their scattered distribution. Our results question homology of 5S rDNA and U1 and U2 snRNA loci in comparative studies. We recommend the use of histone H3 in studies of karyotype evolution.

Patterns of Sex Chromosome Differentiation in Spiders: Insights from Comparative Genomic Hybridisation.

Spiders are an intriguing model to analyse sex chromosome evolution because of their peculiar multiple X chromosome systems. Y chromosomes were considered rare in this group, arising after neo-sex chromosome formation by X chromosome-autosome rearrangements. However, recent findings suggest that Y chromosomes are more common in spiders than previously thought. Besides neo-sex chromosomes, they are also involved in the ancient X1X2Y system of haplogyne spiders, whose origin is unknown. Furthermore, spiders seem to exhibit obligatorily one or two pairs of cryptic homomorphic XY chromosomes (further cryptic sex chromosome pairs, CSCPs), which could represent the ancestral spider sex chromosomes. Here, we analyse the molecular differentiation of particular types of spider Y chromosomes in a representative set of ten species by comparative genomic hybridisation (CGH). We found a high Y chromosome differentiation in haplogyne species with X1X2Y system except for Loxosceles spp. CSCP chromosomes exhibited generally low differentiation. Possible mechanisms and factors behind the observed patterns are discussed. The presence of autosomal regions marked predominantly or exclusively with the male or female probe was also recorded. We attribute this pattern to intraspecific variability in the copy number and distribution of certain repetitive DNAs in spider genomes, pointing thus to the limits of CGH in this arachnid group. In addition, we confirmed nonrandom association of chromosomes belonging to particular CSCPs at spermatogonial mitosis and spermatocyte meiosis and their association with multiple Xs throughout meiosis. Taken together, our data suggest diverse evolutionary pathways of molecular differentiation in different types of spider Y chromosomes.

Absence of W Chromosome in Psychidae Moths and Implications for the Theory of Sex Chromosome Evolution in Lepidoptera.

Moths and butterflies (Lepidoptera) are the largest group with heterogametic females. Although the ancestral sex chromosome system is probably Z0/ZZ, most lepidopteran species have the W chromosome. When and how the W chromosome arose remains elusive. Existing hypotheses place the W origin either at the common ancestor of Ditrysia and Tischeriidae, or prefer independent origins of W chromosomes in these two groups. Due to their phylogenetic position at the base of Ditrysia, bagworms (Psychidae) play an important role in investigating the W chromosome origin. Therefore, we examined the W chromosome status in three Psychidae species, namely Proutiabetulina, Taleporiatubulosa, and Diplodomalaichartingella, using both classical and molecular cytogenetic methods such as sex chromatin assay, comparative genomic hybridization (CGH), and male vs. female genome size comparison by flow cytometry. In females of all three species, no sex chromatin was found, no female-specific chromosome regions were revealed by CGH, and a Z-chromosome univalent was observed in pachytene oocytes. In addition, the genome size of females was significantly smaller than males. Overall, our study provides strong evidence for the absence of the W chromosome in Psychidae, thus supporting the hypothesis of two independent W chromosome origins in Tischeriidae and in advanced Ditrysia.

A chromosome-level genome assembly of Cydia pomonella provides insights into chemical ecology and insecticide resistance.

The codling moth Cydia pomonella, a major invasive pest of pome fruit, has spread around the globe in the last half century. We generated a chromosome-level scaffold assembly including the Z chromosome and a portion of the W chromosome. This assembly reveals the duplication of an olfactory receptor gene (OR3), which we demonstrate enhances the ability of C. pomonella to exploit kairomones and pheromones in locating both host plants and mates. Genome-wide association studies contrasting insecticide-resistant and susceptible strains identify hundreds of single nucleotide polymorphisms (SNPs) potentially associated with insecticide resistance, including three SNPs found in the promoter of CYP6B2. RNAi knockdown of CYP6B2 increases C. pomonella sensitivity to two insecticides, deltamethrin and azinphos methyl. The high-quality genome assembly of C. pomonella informs the genetic basis of its invasiveness, suggesting the codling moth has distinctive capabilities and adaptive potential that may explain its worldwide expansion.

Maleness-on-the-Y (MoY) orchestrates male sex determination in major agricultural fruit fly pests.

In insects, rapidly evolving primary sex-determining signals are transduced by a conserved regulatory module controlling sexual differentiation. In the agricultural pest Ceratitis capitata (Mediterranean fruit fly, or Medfly), we identified a Y-linked gene, Maleness-on-the-Y (MoY), encoding a small protein that is necessary and sufficient for male development. Silencing or disruption of MoY in XY embryos causes feminization, whereas overexpression of MoY in XX embryos induces masculinization. Crosses between transformed XY females and XX males give rise to males and females, indicating that a Y chromosome can be transmitted by XY females. MoY is Y-linked and functionally conserved in other species of the Tephritidae family, highlighting its potential to serve as a tool for developing more effective control strategies against these major agricultural insect pests.

Sex Chromosomes of the Iconic Moth Abraxas grossulariata (Lepidoptera, Geometridae) and Its Congener A. sylvata.

The magpie moth, Abraxas grossulariata, is an iconic species in which female heterogamety was discovered at the beginning of the 20th century. However, the sex chromosomes of this species have not yet been cytologically identified. We describe the sex chromosomes of A. grossulariata and its congener, A. sylvata. Although these species split only around 9.5 million years ago, and both species have the expected WZ/ZZ chromosomal system of sex determination and their sex chromosomes share the major ribosomal DNA (rDNA) representing the nucleolar organizer region (NOR), we found major differences between their karyotypes, including between their sex chromosomes. The species differ in chromosome number, which is 2n = 56 in A. grossularita and 2n = 58 in A. sylvata. In addition, A. grossularita autosomes exhibit massive autosomal blocks of heterochromatin, which is a very rare phenomenon in Lepidoptera, whereas the autosomes of A. sylvata are completely devoid of distinct heterochromatin. Their W chromosomes differ greatly. Although they are largely composed of female-specific DNA sequences, as shown by comparative genomic hybridization, cross-species W-chromosome painting revealed considerable sequence differences between them. The results suggest a relatively rapid molecular divergence of Abraxas W chromosomes by the independent spreading of female-specific repetitive sequences.

New Insights into the Evolution of the W Chromosome in Lepidoptera.

Moths and butterflies (Lepidoptera) represent the most diverse group of animals with heterogametic females. Although the vast majority of species has a WZ/ZZ (female/male) sex chromosome system, it is generally accepted that the ancestral system was Z/ZZ and the W chromosome has evolved in a common ancestor of Tischeriidae and Ditrysia. However, the lack of data on sex chromosomes in lower Lepidoptera has prevented a formal test of this hypothesis. Here, we performed a detailed analysis of sex chromosomes in Tischeria ekebladella (Tischeriidae) and 3 species representing lower Ditrysia, Cameraria ohridella (Gracillariidae), Plutella xylostella (Plutellidae), and Tineola bisselliella (Tineidae). Using comparative genomic hybridization we show that the first 3 species have well-differentiated W chromosomes, which vary considerably in their molecular composition, whereas T. bisselliella has no W chromosome. Furthermore, our results suggest the presence of neo-sex chromosomes in C. ohridella. For Z chromosomes, we selected 5 genes evenly distributed along the Z chromosome in ditrysian model species and tested their Z-linkage using qPCR. The tested genes (Henna, laminin A, Paramyosin, Tyrosine hydroxylase, and 6-Phosphogluconate dehydrogenase) proved to be Z-linked in all species examined. The conserved synteny of the Z chromosome across Tischeriidae and Ditrysia, along with the W chromosome absence in the lower ditrysian families Psychidae and Tineidae, suggests a possible independent origin of the W chromosomes in these 2 lineages.

Chromosomal distribution of interstitial telomeric sequences as signs of evolution through chromosome fusion in six species of the giant water bugs (Hemiptera, Belostoma).

Tandem arrays of TTAGG repeats show a highly conserved location at the telomeres across the phylogenetic tree of arthropods. In giant water bugs Belostoma, the chromosome number changed during speciation by fragmentation of the single ancestral X chromosome, resulting in a multiple sex chromosome system. Several autosome-autosome fusions and a fusion between the sex chromosome pair and an autosome pair resulted in the reduced number in several species. We mapped the distribution of telomeric sequences and interstitial telomeric sequences (ITSs) in Belostoma candidulum (2n = 12 + XY/XX; male/female), B. dentatum (2n = 26 + X1X2Y/X1X1X2X2), B. elegans (2n = 26 + X1X2Y/X1X1X2X2), B. elongatum (2n = 26 + X1X2Y/X1X1X2X2), B. micantulum (2n = 14 + XY/XX), and B. oxyurum (2n = 6 + XY/XX) by FISH with the (TTAGG) n probes. Hybridization signals confirmed the presence of TTAGG repeats in the telomeres of all species examined. The three species with reduced chromosome numbers showed additional hybridization signals in interstitial positions, indicating the occurrence of ITS. From the comparison of all species here analyzed, we observed inverse relationships between chromosome number and chromosome size, and between presence/absence of ITS and chromosome number. The ITS distribution between these closely related species supports the hypothesis that several telomere-telomere fusions of the chromosomes from an ancestral diploid chromosome number 2n = 26 + XY/XX played a major role in the karyotype evolution of Belostoma. Consequently, our study provide valuable features that can be used to understand the karyotype evolution, may contribute to a better understanding of taxonomic relationships, and also elucidate the high plasticity of nuclear genomes at the chromosomal level during the speciation processes.

W-enriched satellite sequence in the Indian meal moth, Plodia interpunctella (Lepidoptera, Pyralidae).

The W chromosome of most lepidopteran species represents the largest heterochromatin entity in the female genome. Although satellite DNA is a typical component of constitutive heterochromatin, there are only a few known satellite DNAs (satDNAs) located on the W chromosome in moths and butterflies. In this study, we isolated and characterized new satDNA (PiSAT1) from microdissected W chromosomes of the Indian meal moth, Plodia interpunctella. Even though the PiSAT1 is mainly localized near the female-specific segment of the W chromosome, short arrays of this satDNA also occur on autosomes and/or the Z chromosome. Probably due to the predominant location in the non-recombining part of the genome, PiSAT1 exhibits a relatively large nucleotide variability in its monomers. However, at least a part of all predicted functional motifs is located in conserved regions. Moreover, we detected polyadenylated transcripts of PiSAT1 in all developmental stages and in both sexes (female and male larvae, pupae and adults). Our results suggest a potential structural and functional role of PiSAT1 in the P. interpunctella genome, which is consistent with accumulating evidence for the important role of satDNAs in eukaryotic genomes.

CpSAT-1, a transcribed satellite sequence from the codling moth, Cydia pomonella.

Satellite DNA (satDNA) is a non-coding component of eukaryotic genomes, located mainly in heterochromatic regions. Relevance of satDNA began to emerge with accumulating evidence of its potential yet hardly comprehensible role that it can play in the genome of many organisms. We isolated the first satDNA of the codling moth (Cydia pomonella, Tortricidae, Lepidoptera), a species with holokinetic chromosomes and a single large heterochromatic element, the W chromosome in females. The satDNA, called CpSAT-1, is located on all chromosomes of the complement, although in different amounts. Surprisingly, the satellite is almost missing in the heterochromatic W chromosome. Additionally, we isolated mRNA from all developmental stages (1st-5th instar larva, pupa, adult), both sexes (adult male and female) and several tissues (Malpighian tubules, gut, heart, testes, and ovaries) of the codling moth and showed the CpSAT-1 sequence was transcribed in all tested samples. Using CpSAT-1 specific primers we amplified, cloned and sequenced 40 monomers from cDNA and gDNA, respectively. The sequence analysis revealed a high mutation rate and the presence of potentially functional motifs, mainly in non-conserved regions of the monomers. Both the chromosomal distribution and the sequence analysis suggest that CPSAT-1 has no function in the C. pomonella genome.

XX/XY System of Sex Determination in the Geophilomorph Centipede Strigamia maritima.

We show that the geophilomorph centipede Strigamia maritima possesses an XX/XY system of sex chromosomes, with males being the heterogametic sex. This is, to our knowledge, the first report of sex chromosomes in any geophilomorph centipede. Using the recently assembled Strigamia genome sequence, we identified a set of scaffolds differentially represented in male and female DNA sequence. Using quantitative real-time PCR, we confirmed that three candidate X chromosome-derived scaffolds are present at approximately twice the copy number in females as in males. Furthermore, we confirmed that six candidate Y chromosome-derived scaffolds contain male-specific sequences. Finally, using this molecular information, we designed an X chromosome-specific DNA probe and performed fluorescent in situ hybridization against mitotic and meiotic chromosome spreads to identify the Strigamia XY sex-chromosome pair cytologically. We found that the X and Y chromosomes are recognizably different in size during the early pachytene stage of meiosis, and exhibit incomplete and delayed pairing.

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.

Neo-sex chromosomes and adaptive potential in tortricid pests.

Changes in genome architecture often have a significant effect on ecological specialization and speciation. This effect may be further enhanced by involvement of sex chromosomes playing a disproportionate role in reproductive isolation. We have physically mapped the Z chromosome of the major pome fruit pest, the codling moth, Cydia pomonella (Tortricidae), and show that it arose by fusion between an ancestral Z chromosome and an autosome corresponding to chromosome 15 in the Bombyx mori reference genome. We further show that the fusion originated in a common ancestor of the main tortricid subfamilies, Olethreutinae and Tortricinae, comprising almost 700 pest species worldwide. The Z-autosome fusion brought two major genes conferring insecticide resistance and clusters of genes involved in detoxification of plant secondary metabolites under sex-linked inheritance. We suggest that this fusion significantly increased the adaptive potential of tortricid moths and thus contributed to their radiation and subsequent speciation.