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.

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► Fosmid-FISH has been applied for the first time to insect chromosomes identification and gene mapping.
► The gene-based comparative FISH mapping revealed the relationship between genomic components of Samia cynthia subspecies with reduced chromosome numbers (2n = 25–28) and Bombyx mori (2n = 56).
► Homologues of either two or four different B. mori chromosomes constitute each chromosome of the S. c. ricini low-number karyotype.
► Our results add another evidence for the internal stability of genomic regions of lepidopteran chromosomes even in species with a low-number karyotype.