Embryologic changes in rabbit lines selected for litter size variability

A divergent selection experiment on litter size variability was carried out. Correlated response in early embryo survival, embryonic development, size of embryos, and size of embryonic coats after four generations of selection was estimated. A total of 429 embryos from 51 high-line females and 648 embryos from 80 low-line females were used in the experiment. The traits studied were percentage of normal embryos, embryo diameter, zona pellucida thickness, and mucin coat thickness. Traits were measured at 24, 48, and 72 hours postcoitum (hpc); mucin coat thickness was only measured at 48 and 72 hpc. The embryos were classified as zygotes or two-cell embryos at 24 hpc; 16-cell embryos or early morulae at 48 hpc; and early morulae, compacted morulae, or blastocyst at 72 hpc. At 24 hpc, the percentage of normal embryos in the high line was lower than in the low line (-2.5%), and embryos in the high line showed 10% higher zona pellucida thickness than those of the low line. No differences in percentage of zygotes or two-cell embryos were found. At 48 hpc, the high-line embryos were less developed, with a higher percentage of 16-cell embryos (23.4%) and a lower percentage of early morulae (-23.4%). At 72 hpc, high-line embryos continued to be less developed, showing higher percentages of early morulae and compact morulae and lower percentages of blastocyst (-1.8%). No differences in embryo diameter or mucin coat thickness were found at any time. In conclusion, selection for litter size variability has consequences on early embryonic survival and development, with embryos presenting a lower state of development and a lower percentage of normal embryos in the line selected for higher variability.