Abstract
Ribosomal DNA clusters and telomeric repeats are important parts of eukaryotic genome. However, little is known about their organization and localization in karyotypes of organisms with holocentric chromosomes. Here we present first cytogenetic study of these molecular structures in seven blue butterflies of the genus Polyommatus Latreille, 1804 with low and high chromosome numbers (from n=10 to n=ca.108) using fluorescence in situ hybridization (FISH) with 18S rDNA and (TTAGG)n telomeric probes. FISH with the 18S rDNA probe showed the presence of two different variants of the location of major rDNA clusters in Polyommatus species: with one or two rDNA-carrying chromosomes in haploid karyotype. We discuss evolutionary trends and possible mechanisms of changes in the number of ribosomal clusters. We also demonstrate that Polyommatus species have the classical insect (TTAGG)n telomere organization. This chromosome end protection mechanism probably originated de novo in small chromosomes that evolved via fragmentations.
Highlights
Most studied butterfly families and genera share the modal chromosome number of n=30 or n=31 (Robinson 1971) and this, most likely ancestral chromosome number is maintained in the Lepidoptera karyotype evolution (Suomalainen 1979, Lukhtanov 2000, 2014)
Using specimens with dramatically different high and low chromosomal numbers we aim to examine the association between karyotype and rDNA cluster number
We investigated distribution of ribosomal clusters in karyotypes by mapping 18S ribosomal DNA probe on chromosomes of P. (A.) caeruleus (Staudinger, 1871), P. (A.) hamadanensis, P. (A.) karindus (Riley, 1921), P. (A.) morgani (Le Cerf, 1909), P. (A.) peilei (Bethune-Baker, 1921), P. (A.) pfeifferi (Brandt, 1938) and P. (A.) sennanensis which are drastically different in their chromosome numbers
Summary
Most studied butterfly families and genera share the modal chromosome number of n=30 or n=31 (Robinson 1971) and this, most likely ancestral chromosome number is maintained in the Lepidoptera karyotype evolution (Suomalainen 1979, Lukhtanov 2000, 2014). Numerous inter- or intrachromosomal rearrangements such as translocations and inversions, can contribute to karyotype evolution without significant changes in chromosome number and size. Detecting these rearrangements is difficult due to several specific properties of Lepidoptera karyotype. Lepidoptera and their sister group, caddisflies (Trichoptera), have holocentric chromosomes, i.e. chromosomes without localized centromeres (Wolf et al 1997), and this makes impossible using the centromere as a marker. Attempts to use differential banding techniques have appeared but were inefficient (Guerra et al 2010)
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