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Abstract
The chromosome complements of ova from gilts selected for high ovulation rate (RS line, Ovulation rate = 15.7) were compared to those from gilts from a control (C, Ovulation rate = 13.4) line. From lymphocyte karyotypic analysis of these lines, it was estimated that the incidence of chromosomal abnormalities in the pigs was less than 2.7%. Within each line, ova chromosome complements from untreated females (U) were compared to those from super-ovulated females (S, Ovulation rate increased 140%). Bilateral salpingectomies were performed on 113 gilts post-ovulation. Removed oviducts were flushed to recover the ova which were then fixed on microscope slides by a new method (previously unpublished), examined, and classified as mature or immature, and then as chromosomally normal, abnormal, or nonanalyzable. With the new fixation method, 53.3% of the ova with chromosomes were analyzable. Analysis of the processing procedure revealed the extreme importance of minimizing the total time required for fixation of the ova, as well as the need to hold it constant across all treatments.
The median percentage of immature ova was greater in both RS line (12.5%) and S gilts (12.8%) than in the C line (4.5%) and U gilts (4.5%), respectively. Within the untreated animals, the median percentage of chromosomally abnormal ova (P-abnormal) was larger in RS (22.2%) than C line (14.3%) gilts, however there was no difference between lines in the superovulated pigs. Within the C line gilts, the median P-abnormal was greater in superovulated (25.9%) than untreated animals (14.3%), however superovulation did not increase P-abnormal in the RS line (U = 28.0%); (S = 22.2%). Of the abnormal ova, 94.5% were aneuploid. Ova immaturity and chromosomal abnormalities may explain 89.7% of the embryo mortality (EM) that was observed in the RS line (EM = 20.4%) prior to 30 days of gestation, compared to 42.0% of the loss in C line gilts (EM = 16.9%). Increasing ovulation rate in gilts by either genetic selection or superovulation appears to decrease the proportion of normal ova.
