Abstract
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.
Highlights
Moths and butterflies (Lepidoptera) constitute, with nearly 160,000 described species, one of the largest groups of animals [1]
Mitotic Karyotypes Chromosome numbers were determined from mitotic metaphase chromosomes prepared from wing imaginal discs, which have a high mitotic index in the last larval instar because of intensive proliferation of the cells
Based on repeated counts we concluded that three species, G. molesta (Figure 1a, b), G. funebrana (Figure 1c, d), and L. botrana (Figure 1e, f), have identical numbers of 2n = 56 chromosomes like the codling moth, C. pomonella, whereas the karyotype of E. ambiguella consists of a higher chromosome number of 2n = 60 (Figure 1g, h)
Summary
Moths and butterflies (Lepidoptera) constitute, with nearly 160,000 described species, one of the largest groups of animals [1]. The Lepidoptera are far more homogeneous, structurally and ecologically, than the other large insect orders such as Coleoptera, Diptera, and Hymenoptera [2]. Holokinetic chromosomes of Lepidoptera possess very few differentiating features They lack primary constrictions (centromeres sensu stricto [3]) and they are usually small, numerous, and uniform in shape. Their holokinetic structure is expected to facilitate karyotype evolution via chromosome fusion and fission (see discussion in [4]), the architecture of lepidopteran genomes appears to be relatively stable. The high degree of conservation at the chromosomal level across the phylogenetic tree of Lepidoptera contrasts with exceptional diversity found in some taxa [15,16]
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