Chromosome Evolution in the Free-Living Flatworms: First Evidence of Intrachromosomal Rearrangements in Karyotype Evolution of Macrostomum lignano (Platyhelminthes, Macrostomida).

Kira Zadesenets,Eugene Berezikov,Nikita Ershov,Nikolay Rubtsov

doi:10.3390/genes8110298

Kira Zadesenets, Eugene Berezikov + Show 2 more

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https://doi.org/10.3390/genes8110298

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Abstract

The free-living flatworm Macrostomum lignano is a hidden tetraploid. Its genome was formed by a recent whole genome duplication followed by chromosome fusions. Its karyotype (2n = 8) consists of a pair of large chromosomes (MLI1), which contain regions of all other chromosomes, and three pairs of small metacentric chromosomes. Comparison of MLI1 with metacentrics was performed by painting with microdissected DNA probes and fluorescent in situ hybridization of unique DNA fragments. Regions of MLI1 homologous to small metacentrics appeared to be contiguous. Besides the loss of DNA repeat clusters (pericentromeric and telomeric repeats and the 5S rDNA cluster) from MLI1, the difference between small metacentrics MLI2 and MLI4 and regions homologous to them in MLI1 were revealed. Abnormal karyotypes found in the inbred DV1/10 subline were analyzed, and structurally rearranged chromosomes were described with the painting technique, suggesting the mechanism of their origin. The revealed chromosomal rearrangements generate additional diversity, opening the way toward massive loss of duplicated genes from a duplicated genome. Our findings suggest that the karyotype of M. lignano is in the early stage of genome diploidization after whole genome duplication, and further studies on M. lignano and closely related species can address many questions about karyotype evolution in animals.

Highlights

Molecular cytogenetic studies performed on the free-living flatworm Macrostomum lignano revealed extended homologous chromosome regions, suggesting hidden tetraploidy, which arose as a result of the whole genome duplication (WGD) followed by chromosome rearrangements [1]
Whole genome duplication is an evolutionary event that played an important role in the diversification of most eukaryotic lineages [15,16,17]
The study of this stage requires comparative genomic analysis in species belonging to taxa containing species that recently underwent WGD and species that did not undergo WGD

Summary

Introduction

Molecular cytogenetic studies performed on the free-living flatworm Macrostomum lignano revealed extended homologous chromosome regions, suggesting hidden tetraploidy, which arose as a result of the whole genome duplication (WGD) followed by chromosome rearrangements [1].Comparison of karyotypes in closely related species indicated the relatively recent WGD that took place in the M. lignano evolutionary lineage. Molecular cytogenetic studies performed on the free-living flatworm Macrostomum lignano revealed extended homologous chromosome regions, suggesting hidden tetraploidy, which arose as a result of the whole genome duplication (WGD) followed by chromosome rearrangements [1]. The largest chromosome (MLI1) was formed through the fusions of one chromosome set of the hypothetical ancestral karyotype, while others probably avoided drastic reorganization. The modern M. lignano karyotype consists of two parts containing similar DNA content, but differing in the number and morphology of their chromosomes. The only difference in DNA content discovered between MLI1 and the set of three pairs of small metacentrics was the loss of two clusters of pericentric and two clusters of telomeric repeats in the chromosome. Some copies of these DNA repeats were conserved in MLI1, but a low number

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