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

Area of Science:

• Evolutionary biology
• Genomics
• Cytogenetics

Background:

• Organismal evolution involves periods of chromosomal conservatism punctuated by rapid chromosomal changes (megaevolution).
• Blue butterflies (Lycaenidae) provide a model to study these evolutionary dynamics.

Purpose of the Study:

• Investigate chromosomal conservatism and megaevolution in blue butterflies using comparative genomic analysis.
• Elucidate the mechanisms driving rapid increases in chromosome number and the role of sex chromosomes.

Main Methods:

• Comparative analysis of chromosome-level genome assemblies in Lycaenidae.
• Karyotype analysis to identify chromosomal rearrangements and breakpoints.
• Investigation of interstitial telomere sequences (ITSs) and their distribution.

Main Results:

• Chromosomal conservatism is linked to stable autosomes and dynamic Z chromosome evolution (NeoZ formation via fusions).
• Rapid chromosomal evolution (megaevolution) primarily occurs through chromosomal fissions, a non-random, canalized process.
• Independent Lysandra lineages show parallel increases in fragmented chromosomes, reusing ancestral breakpoints.
• No evidence of polyploidy; interstitial telomere sequences (ITSs) are present in rapidly evolving karyotypes but absent in ancestral ones.
• The Z sex chromosome plays a significant role, potentially enhanced by fusions and inversions.

Conclusions:

• Chromosomal megaevolution in blue butterflies is a non-random, canalized process.
• Transposition of telomeric sequences may trigger rapid chromosome number increases.
• The Z sex chromosome is disproportionally involved in chromosomal evolution, influenced by fusions and inversions.