Rapid Selection, Overdominance & Disease As A Side Effect Of Adaptation: Domesticated Sheep And Twin Births
Multiple ovulations are uncommon in most breeds of sheep. Presumably this was adaptive among the wild ancestors of sheep, since it is considerably harder to feed and protect twins. Singleton births were probably the optimal course until recent years, when some sheep farmers have created far more favourable environments than existed in the past. In such environments, twinning is favoured by selection, and mutations favouring twinning have reached polymorphic frequencies in some breeds of sheep. Inverdale sheep (Galloway et al., 2000) carry a naturally occurring X-linked mutation of the BMP15 gene (bone morphological protein 15, a growth and differentiation factor expressed in ovaries) which causes increased ovulation rate and twin and triplet births in heterozygotes, but sterility due to primary ovarian failure in homozygotes. Hanna sheep carry a different mutation in BMP15 with similar effects. It is interesting that in these cases, the very function that is hypertrophied in heterozygotes and favoured by selection is completely destroyed in homozygotes. Booroola sheep (McNatty et al., 2001) carry a mutation in the receptor of BMP-1B, another member of the bone morphological protein family. In this case ovulation is increased even further in homozygotes. Note the tendency of selection to affect genes that are closely biochemically related because they lead to similar outcomes.
Galloway, S. M., McNatty, K. P., Cambridge, L. M., Laitinen, M. P., Juengel, J. L., Jokirata, T. S. et al. (2000) Mutations in an oocyte-derived growth factor gene (BMP15) cause increased ovulation rate and infertility in a dosage-sensitive manner. Nature Genetics 25, 279–283.
McNatty, K. P., Juengel, J. L., Wilson, T., Galloway, S. M. & Davis, G. H. (2001) Genetic mutations influencing ovulation rate in sheep. Reproduction Fertility and Development 13, 549–555.
Quoted on Sun May 20th, 2012