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Mol Biol Evol. 2015 Jan;32(1):81-90. doi: 10.1093/molbev/msu268. Epub 2014 Sep 18.

Sustained heterozygosity across a self-incompatibility locus in an inbred ascidian.

Author information

1
Department of Zoology, Graduate School of Science, Kyoto University, Sakyo, Kyoto, Japan yutaka@ascidian.zool.kyoto-u.ac.jp.
2
Department of Zoology, Graduate School of Science, Kyoto University, Sakyo, Kyoto, Japan.
3
Shimoda Marine Research Center, University of Tsukuba, Shimoda, Shizuoka, Japan.
4
Marine Genomics Unit and DNA Sequencing Section, Okinawa Institute of Science and Technology Graduate University, Onna, Okinawa, Japan.
5
Graduate School of Information Science and Technology, Hokkaido University, Sapporo, Japan.
6
Department of Zoology, Graduate School of Science, Kyoto University, Sakyo, Kyoto, Japan Marine Genomics Unit and DNA Sequencing Section, Okinawa Institute of Science and Technology Graduate University, Onna, Okinawa, Japan.

Abstract

Because self-incompatibility loci are maintained heterozygous and recombination within self-incompatibility loci would be disadvantageous, self-incompatibility loci are thought to contribute to structural and functional differentiation of chromosomes. Although the hermaphrodite chordate, Ciona intestinalis, has two self-incompatibility genes, this incompatibility system is incomplete and self-fertilization occurs under laboratory conditions. Here, we established an inbred strain of C. intestinalis by repeated self-fertilization. Decoding genome sequences of sibling animals of this strain identified a 2.4-Mbheterozygous region on chromosome 7. A self-incompatibility gene, Themis-B, was encoded within this region. This observation implied that this self-incompatibility locus and the linkage disequilibrium of its flanking region contribute to the formation of the 2.4-Mb heterozygous region, probably through recombination suppression. We showed that different individuals in natural populations had different numbers and different combinations of Themis-B variants, and that the rate of self-fertilization varied among these animals. Our result explains why self-fertilization occurs under laboratory conditions. It also supports the concept that the Themis-B locus is preferentially retained heterozygous in the inbred line and contributes to the formation of the 2.4-Mb heterozygous region. High structural variations might suppress recombination, and this long heterozygous region might represent a preliminary stage of structural differentiation of chromosomes.

KEYWORDS:

Ciona intestinalis; chromosome evolution; inbred line; self-incompatibility

PMID:
25234703
DOI:
10.1093/molbev/msu268
[Indexed for MEDLINE]

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