학술논문
Origin of a Giant Sex Chromosome
Document Type
Academic Journal
Author
Source
Molecular Biology and Evolution. April 2021, Vol. 38 Issue 4, p1554, 16 p.
Subject
Language
English
ISSN
0737-4038
Abstract
Introduction Almost two centuries of cytogenetic studies have revealed the great diversity of animal karyotypes. Chromosome numbers range from a single chromosome pair in the jack jumper ant (Crosland and [...]
Chromosome size and morphology vary within and among species, but little is known about the proximate or ultimate causes of these differences. Cichlid fish species in the tribe Oreochromini share an unusual giant chromosome that is ~3 times longer than the other chromosomes. This giant chromosome functions as a sex chromosome in some of these species. We test two hypotheses of how this giant sex chromosome may have evolved. The first hypothesis proposes that it evolved by accumulating repetitive elements as recombination was reduced around a dominant sex determination locus, as suggested by canonical models of sex chromosome evolution. An alternative hypothesis is that the giant sex chromosome originated via the fusion of an autosome with a highly repetitive B chromosome, one of which carried a sex determination locus. We test these hypotheses using comparative analysis of chromosome-scale cichlid and teleost genomes. We find that the giant sex chromosome consists of three distinct regions based on patterns of recombination, gene and transposable element content, and synteny to the ancestral autosome. The WZ sex determination locus encompasses the last ~105 Mb of the 134-Mb giant chromosome. The last 47 Mb of the giant chromosome shares no obvious homology to any ancestral chromosome. Comparisons across 69 teleost genomes reveal that the giant sex chromosome contains unparalleled amounts of endogenous retroviral elements, immunoglobulin genes, and long noncoding RNAs. The results favor the B chromosome fusion hypothesis for the origin of the giant chromosome. Key words: sex chromosomes, genome evolution, chromosome evolution, supernumerary chromosome, chromosome fusion, genetic conflict.
Chromosome size and morphology vary within and among species, but little is known about the proximate or ultimate causes of these differences. Cichlid fish species in the tribe Oreochromini share an unusual giant chromosome that is ~3 times longer than the other chromosomes. This giant chromosome functions as a sex chromosome in some of these species. We test two hypotheses of how this giant sex chromosome may have evolved. The first hypothesis proposes that it evolved by accumulating repetitive elements as recombination was reduced around a dominant sex determination locus, as suggested by canonical models of sex chromosome evolution. An alternative hypothesis is that the giant sex chromosome originated via the fusion of an autosome with a highly repetitive B chromosome, one of which carried a sex determination locus. We test these hypotheses using comparative analysis of chromosome-scale cichlid and teleost genomes. We find that the giant sex chromosome consists of three distinct regions based on patterns of recombination, gene and transposable element content, and synteny to the ancestral autosome. The WZ sex determination locus encompasses the last ~105 Mb of the 134-Mb giant chromosome. The last 47 Mb of the giant chromosome shares no obvious homology to any ancestral chromosome. Comparisons across 69 teleost genomes reveal that the giant sex chromosome contains unparalleled amounts of endogenous retroviral elements, immunoglobulin genes, and long noncoding RNAs. The results favor the B chromosome fusion hypothesis for the origin of the giant chromosome. Key words: sex chromosomes, genome evolution, chromosome evolution, supernumerary chromosome, chromosome fusion, genetic conflict.