Production of lambs of predetermined sex after the insemination of ewes with low numbers of frozen–thawed sorted X- or Y-chromosome-bearing spermatozoa

Abstract

The fertilizing ability of sex-sorted frozen–thawed ram spermatozoa was assessed after insemination of mature Merino ewes at a synchronized oestrus. Ewes were inseminated into the uterus or utero–tubal junction (UTJ) with a total of 140 × 10⁶ unsorted (control) or 2–4 × 10⁶ sorted (X or Y) frozen–thawed ram spermatozoa 54 to 57 hours after removal of progestagen-impregnated pessaries and an injection of 400 IU of pregnant mare serum gonadotrophin (Folligon^®, Intervet). The spermatozoa were separated into X- and Y-chromosome-bearing spermatozoa after analysis with a modified high-speed cell sorter (SX MoFlo^®). The number of ewes pregnant after insemination with unsorted frozen–thawed spermatozoa was significantly higher (26/48; 54.3%) than for ewes inseminated with either X- (12/48; 25.0%) or Y-sorted spermatozoa (7/48; 14.6%) (P<0.05). Seventeen of the eighteen lambs produced by ewes inseminated with X-sorted spermatozoa were female (94.4%) and 8/8 lambs from ewes inseminated with Y-sorted spermatozoa were male (100%). The sex ratio of the lambs born to ewes inseminated with sex-sorted spermatozoa was significantly skewed from the 51.3% male and 48.7% female ratio in the control group (P<0.05). This study showed, for the first time, that lambs of predicted sex can be produced after insemination with low numbers of sex-sorted cryopreserved ram spermatozoa.