Dynamic meiotic behavior and evolutionary insights of supernumerary B chromosomes in the hangingfly Bittacus cirratus (Mecoptera, Bittacidae)

By

He, Le-Le
Miao, Ying.
Wang, He-Hong
Zhang, Jie
Hua, Baozhen

Type

Article

Date of Publication

2025-6-4

Original Publication

Comparative cytogenetics : international journal of plant & animal cytogenetics, karyosystematics and molecular systematics

Volume

19

Pages

91--107

Subjects

chromosome rearrangement , Cytogenetics , heteromorphic homologues , karyotype , meiotic drive

Contributed by

Pensoft Publishers

Language

English

Published by

St. Petersburg, Russia [Published] with technical support of Zoological Institute RAS [2007]-

Identifiers

DOI: https://doi.org/10.3897/compcytogen.19.153340

 

Abstract

Supernumerary B chromosomes are significant dispensable genetic elements that follow their own species-specific evolutionary pathways. Despite their widespread occurrence, comprehensive analyses of their meiotic behavior remain limited. In this study, we present the first systematic investigation of B chromosome morphology and meiotic behavior in the hangingfly Bittacus cirratus Tjeder, 1956 using cytogenetic approaches. The male basal chromosome numbers of B. cirratus is 2n = 30 + XO, with 0–5 polymorphic B chromosomes. Intraspecific B chromosome polymorphism manifests as various distinct morphotypes ranging from punctiform, bicentric, and ring-shaped to larger coiled forms, indicating that the B chromosomes may undergo rapid structural changes. During meiosis, B chromosomes display transmission drive through asymmetric segregation, preferentially accumulating in one daughter cell. Most B chromosomes formed univalents, with few forming bivalents or trivalents at meiosis I. Three unconventional retention mechanisms were identified in univalent B chromosomes: (1) associating with a nonhomologous chromosome, (2) accumulating near spindle poles, and (3) contributing to unequal spindle formation. Based on the abundant chromosomal changes of A chromosomes and stable XX/XO sex determination, we infer that the B chromosomes likely originated from multiple A chromosomes in B. cirratus. The roles of B chromosomes in the cell cycle and individual fitness are briefly discussed, and the evolutionary scenario is putatively put forward for the diversification of B chromosomes.