Chromosomal conservatism vs chromosomal megaevolution: enigma of karyotypic evolution in Lepidoptera

Polyommatine blue butterflies reveal unexpected integrity of the W sex chromosome amid extensive chromosome fragmentation

“…Indeed, in B. mori, it was shown that ends of chromosome fragments induced by X-ray irradiation were healed by de novo addition of (TTAGG) n repeats, which are important for their stable transmision (Fujiwara et al 2000). We failed to reliably detect any autosomal interstitial telomeric sequences reported in L. bellargus and L. coridon (Pazhenkova and Lukhtanov 2023), which is not surprising as the length of these ITS seems to be <1.5 Kb (Pazhenkova and Lukhtanov 2023), which is below detection threshold of conventional FISH protocols (Speel et al 2000, Carabajal Paladino et al 2014).…”

“…Mora et al 2024). Providing that differences in genome sizes are determined by repetitive elements (Pazhenkova and Lukhtanov 2023), increase in genome sizes in species with fragmented karyotypes supports an instrumental role of mobile elements in karyotypic diversification of Lysandra and Polyommatus spp. However, even in our small sample, we also observed considerable differences in genome sizes between and within populations in P. icarus and P. fulgens (Table I).…”

“…Based on two- and fourfold differences in chromosome numbers observed between closely related lycaenid species such as P. icarus (n = 22–23), L. bellargus (n = 45), and L. coridon (n = 87–97), polyploidy was hypothesized as a possible mode of karyotype evolution in these butterflies. Polyploidy in lycaenids has been previously refuted by indirect evidence such as comparison of areas occupied by native metaphase I plates or negative correlarion between number of chromosomes and their size (Kandul et al 2007, Pazhenkova and Lukhtanov 2023). More recently, length of genome assemblies as well as gene syntenies favored chromosome fragmentation over whole genome duplications in L. bellargus and L. coridon (Lohse et al 2022, Vila et al 2023, Pazhenkova and Lukhtanov 2023, Wright et al 2024).…”

“…Polyploidy in lycaenids has been previously refuted by indirect evidence such as comparison of areas occupied by native metaphase I plates or negative correlarion between number of chromosomes and their size (Kandul et al 2007, Pazhenkova and Lukhtanov 2023). More recently, length of genome assemblies as well as gene syntenies favored chromosome fragmentation over whole genome duplications in L. bellargus and L. coridon (Lohse et al 2022, Vila et al 2023, Pazhenkova and Lukhtanov 2023, Wright et al 2024). Our results provide first genome size values determined by flow cytometry for representatives of all three polyommatine lineages with high chromosome numbers.…”

“…Indeed, comparative genomics show that the sex chromosome Z remains intact in lineages with extensive chromosome fragmentation (Wright et al 2024). Moreover, recent studies have shown that unlike in female heterogametic vertebrates (Pennell et al 2015), sex chromosome-autosome fusions are quite common in Lepidoptera including representatives of the family Lycaenidae (Carabajal Paladino et al 2019, Pazhenkova and Lukhtanov 2023; Wright et al 2024) and these sex chromosome turnovers may be driven by sexual antagonistic selection (Mora et al 2024). Yet, sex chromosomes remain generally understudied in female heterogametic taxa (Ellegren 2011, Tomaszkiewicz et al 2017) and particularly female limited W chromosomes are not well represented in lepidopteran chromosome-level assemblies due to their high repeat content and lower sequencing depth compared to autosomes (Carey et al 2022, Wright et al 2024).…”