Sex of co-twin affects the in vitro developmental competence of oocytes derived from 6- to 8-week-old lambs

Abstract

Several intrinsic factors (age, genotype, liveweight) influence the reliability of juvenile in vitro fertilisation embryo transfer (JIVET) programs. Limited evidence indicates that variability between lambs is reduced in twin-born lambs. We examined the impact of birth type (single, twin, triplet) and sex of the co-twin (with age, birthweight and liveweight as covariates) on JIVET outcomes. Birth type did not influence any parameter studied. However, blastocysts produced, as a percentage of embryos cleaved or total cumulus–oocyte complexes collected, was higher (P < 0.05) for females born with a female co-twin (67.0 ± 6.1, 57.5 ± 6.0 respectively) compared with those born with a male co-twin (26.9 ± 6.5, 22.3 ± 6.2 respectively; least-square mean ± s.e.m.). Blastocyst rates for lambs born with a male co-twin did not differ significantly from lambs born either as singles (39.5 ± 6.7%, 34.6 ± 6.5% respectively) or triplets (43.1 ± 10.6%, 36.5 ± 10.3% respectively). Other parameters were not influenced by sex of the co-twin. These results are indicative of an enhancement effect of the female co-twin on oocyte development. From a practical perspective, selecting lambs for a JIVET program based on litter size and sex of the co-twin is warranted.