Positive selection drives faster-Z evolution in silkmoths.
Genes linked to X or Z chromosomes, which are hemizygous in the heterogametic sex, are predicted to evolve at different rates than those on autosomes. This "faster-X effect" can arise either as a consequence of hemizygosity, which leads to more efficient selection for recessive beneficial mutations in the heterogametic sex, or as a consequence of reduced effective population size of the hemizygous chromosome, which leads to increased fixation of weakly deleterious mutations due to genetic drift. Empirical results to date suggest that, while the overall pattern across taxa is complicated, systems with male heterogamy show a faster-X effect attributable to more efficient selection, whereas the faster-Z effect in female-heterogametic taxa is attributable to increased drift. To test the generality of the faster-Z pattern seen in birds and snakes, we sequenced the genome of the lepidopteran silkmoth Bombyx huttoni. We show that silkmoths experience faster-Z evolution, but unlike in birds and snakes, the faster-Z effect appears to be attributable to more efficient positive selection. These results suggest that female heterogamy alone is unlikely to explain the reduced efficacy of selection on vertebrate Z chromosomes. It is likely that many factors, including differences in overall effective population size, influence Z chromosome evolution.
Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts 02138. tsackton@oeb.harvard.edu.
Timothy B
TB
Sackton
Russell B
RB
Corbett-Detig
Javaregowda
J
Nagaraju
Lakshmi
L
Vaishna
Kallare P
KP
Arunkumar
Daniel L
DL
Hartl
D000818:Animals; D012831:Bombyx / Q000235:genetics*; D002874:Chromosome Mapping; D003062:Codon; D019143:Evolution, Molecular; D005260:Female; D049750:Genome, Insect; D016014:Linear Models; D008297:Male; D012641:Selection, Genetic; D017422:Sequence Analysis, DNA; D012730:Sex Chromosomes / Q000235:genetics*
D003062:Codon
Bombyx mori; sex chromosome evolution