# Chromosomal Inversions in Chromosome U of Drosophila subobscura: A Story from Population Studies to Molecular Level

## TL;DR

This study explores chromosomal inversions in Drosophila subobscura, revealing how transposons may have contributed to their formation and identifying a new genetic element.

## Contribution

The paper identifies transposon-mediated origins of two inversions and discovers a new fold-back element in Drosophila subobscura.

## Key findings

- The U6 and U8 inversions likely originated via transposons rather than non-homologous end-joining.
- A new fold-back-like element named Ziga-Zaga was identified at the distal breakpoint of inversion U8.
- Breakpoints of U6 and U8 were localized using genomic mapping techniques.

## Abstract

Drosophila subobscura is a Palearctic species that colonized the west coast of both Americas in the last quarter of the 20th century. This species stands out for its large chromosomal inversion polymorphism that affects its five long chromosomes. Studies of natural populations in these three geographic areas revealed that the inversion polymorphism has an adaptive character. However, there is no information on which genes may be involved in the different climatic adaptations. Characterization of the inversion breakpoints will allow a first approach to the genes included in the inversions and to find candidates to be affected by selection. In this work, we take advantage of the existence of a reference genomic sequence that carries the U1+2 arrangement to localize the breakpoints of inversions U6 and U8. Our results suggest that the origin of both inversions would have been mediated by transposons and, in the case of U8, we found a new fold-back-type element characteristic of the subobscura species.

Drosophila subobscura is a Palearctic species that colonized the west coast of South and North America in the last quarter of the 20th century. This species stands out for its large chromosomal inversion polymorphism that affects its five long chromosomes. Studies of natural populations revealed that the inversion polymorphism has an adaptive character and while the arrangement Ust was classified as adapted to cold, other arrangements, such as U1+2+6 and U1+8+2, were considered warm adapted. Characterization of the inversion breakpoints will allow a first approach to the genes included in the inversions and to find candidates to be affected by selection. In this work, we take advantage of the existence of a reference genomic sequence carrying the U1+2 arrangement to locate the breakpoints of the U6 and U8 inversions, mapping paired-end Illumina reads from two homokaryotypic strains to U1+2+6 and U1+8+2, respectively. To date, most of the characterized inversion breakpoints in D. subobscura have been generated by non-homologous end-joining. In contrast, the U6 and U8 inversions seem to have originated by transposons, and, at the distal breakpoint of inversion U8, we describe a new fold-back-like element characteristic of the suboscura species group that we have named Ziga-Zaga.