Abstract
Karyotype analysis of nine strains of the peach-potato aphid Myzus persicae (Sulzer, 1776), collected on Lavandula sp. plants, evidenced showed that five of them had a standard 2n = 12 karyotype, one possessed a fragmentation of the X chromosome occurring at the telomere opposite to the NOR-bearing one and three strains had a chromosome number 2n = 11 due to a non-reciprocal translocation of an autosome A3 onto an A1 chromosome. Interestingly, the terminal portion of the autosome A1 involved in the translocation was the same in all the three strains, as evidenced by FISH with the histone cluster as a probe. The study of telomeres in the Myzus persicae strain with the X fission evidenced that telomerase synthesised de novo telomeres at the breakpoints resulting in the stabilization of the chromosomal fragments. Lastly, despite the presence of a conserved telomerase, aphid genome is devoid of genes coding for shelterin, a complex of proteins involved in telomere functioning frequently reported as conserved in eukaryotes. The absence of this complex, also confirmed in the genome of other arthropods, suggests that the shift in the sequence of the telomeric repeats has been accompanied by other changes in the telomere components in arthropods in respect to other metazoans.
Highlights
Karyotype features are usually stable within species, and chromosomal changes, if they occur, contribute to the formation of a post-zygotic barrier between biological populations causing the establishment of reproductive isolation and speciation as a possible consequence (Noor et al 2001, Delneri et al 2003, Kandul et al 2007)
Holocentric chromosomes have been frequently described as a powerful tool to stabilize and inherit chromosomal mutations resulting in karyotype changes (Monti et al 2012a, 2012b)
Even if it is not clear if the observed karyotype variants have phenotypic effects over short temporal and spatial scales on aphid evolution and adaptation, the presence of chromosomal fissions and fusions could allow a rapid karyotype evolution at fine geographic scales so that aphid species could be the sum of populations that can have different karyotypes that in turn can give diverse genetic/ecological/evolutionary responses in relation to imposed selective environmental forces (Monti et al 2012a, 2012b)
Summary
Karyotype features are usually stable within species, and chromosomal changes, if they occur, contribute to the formation of a post-zygotic barrier between biological populations causing the establishment of reproductive isolation and speciation as a possible consequence (Noor et al 2001, Delneri et al 2003, Kandul et al 2007). Kandul et al 2007) Despite these general rules, the speciation models were still problematic since numerous cases of intraspecific karyotype instability have been described in literature and at present the most extreme case was published by Lukhtanov et al (2011) reporting in the butterfly Leptidea sinapis (Linnaeus, 1758) the first clearly documented example of explosive chromosome number evolution through intraspecific and intrapopulation accumulation of multiple chromosomal changes. An intriguing exception is represented by the large genus Agrodiaetus (Hübner, 1822) (Lepidoptera: Lycaenidae), which exhibits an unusual interspecific diversity in chromosome number, from n = 10 to 134, allowing to assess that a rapid karyotypic diversification is likely to have contributed to this explosive speciation rate (Kandul et al 2007)
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