The master sex-determination locus in threespine sticklebacks is on a nascent Y chromosome.
BACKGROUND
Many different environmental and genetic sex-determination mechanisms are found in nature. Closely related species can use different master sex-determination switches, suggesting that these developmental pathways can evolve very rapidly. Previous cytological studies suggest that recently diverged species of stickleback fish have different sex chromosome complements. Here, we investigate the genetic and chromosomal mechanisms that underlie sex determination in the threespine stickleback (Gasterosteus aculeatus).


RESULTS
Genome-wide linkage mapping identifies a single chromosome region at the distal end of linkage group (LG) 19, which controls male or female sexual development in threespine sticklebacks. Although sex chromosomes are not cytogenetically visible in this species, several lines of evidence suggest that LG 19 is an evolving sex chromosome system, similar to the XX female/XY male system in many other species: (1) males are consistently heterozygous for unique alleles in this region; (2) recombination between loci linked to the sex-determination region is reduced in male meiosis relative to female meiosis; (3) sequence analysis of X- and Y-specific bacterial artificial chromosome (BAC) clones from the sex-determination region reveals many sequence differences between the X- and Y-specific clones; and (4) the Y chromosome has accumulated transposable elements and local duplications.


CONCLUSIONS
Taken together, our data suggest that threespine sticklebacks have a simple chromosomal mechanism for sex determination based on a nascent Y chromosome that is less than 10 million years old. Further analysis of the stickleback system will provide an exciting window into the evolution of sex-determination pathways and sex chromosomes in vertebrates.
Department of Developmental Biology and Howard Hughes Medical Institute, Stanford University School of Medicine, Palo Alto, CA 94305, USA. cpeichel@fhcrc.org
Catherine L
CL
Peichel
Joseph A
JA
Ross
Clinton K
CK
Matson
Mark
M
Dickson
Jane
J
Grimwood
Jeremy
J
Schmutz
Richard M
RM
Myers
Seiichi
S
Mori
Dolph
D
Schluter
David M
DM
Kingsley
D000818:Animals; D001483:Base Sequence; D002874:Chromosome Mapping; D022202:Chromosomes, Artificial, Bacterial; D016000:Cluster Analysis; D003433:Crosses, Genetic; D017931:DNA Primers; D030561:Databases, Nucleic Acid; D019143:Evolution, Molecular; D005260:Female; D008297:Male; D008969:Molecular Sequence Data; D010802:Phylogeny; D011995:Recombination, Genetic; D012091:Repetitive Sequences, Nucleic Acid / Q000235:genetics; D016415:Sequence Alignment; D017422:Sequence Analysis, DNA; D017385:Sequence Homology; D019849:Sex Determination Processes; D023701:Smegmamorpha / Q000235:genetics*; D014998:Y Chromosome / Q000235:genetics*
D017931:DNA Primers