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Offspring sex-ratio and reproductive performance in heterogametic females of the South American field mouse Akodon azarae. Reproduction in heterogametic Akodon azarae females.

We have compared the breeding performance of homogametic (XX) and heterogametic (XY*) females of the South American sigmodontine rodent Akodon azarae under laboratory conditions. XY* females showed an enhanced reproductive performance when compared with normal, XX, females. The XY* females had a longer reproductive lifespan. They started to reproduce early, had more frequent litters, and stopped reproduction later than XX females. Their progeny showed a biased 1:2 male:female sex ratio which may be explained by the early loss of YY* zygotes after fertilization. However, litter size at birth was similar both in XY* and XX females, and no difference in ovulation rate was detected between them. This indicates that an "automatic" rather than an "evolved" reproductive compensation mechanism may be acting in heterogametic females. A separate study has shown that self-synapsis of both the X and Y* chromosomes takes place during meiosis, allowing the oocytes to escape from functional deterioration. It is suggested that self-synapsis and "automatic" reproductive compensation account for the preservation of fertility in heterogametic females in spite of the heteromorphic sex chromosomes and the early embryo loss they experience. However, these mechanisms do not account for the enhancement of reproductive lifespan. The possibility that an intrauterine position phenomenon is acting in A. azarae is discussed.