Correlated responses to selection for large body size in oMt1a-oGH transgenic mice: reproductive traits

Correlated responses in female reproductive performance were evaluated foll owing short-term selection within full-sib families for increased 8-week bo dy weight in two replicates of four lines of mice: two ovine metallothionei n-ovine growth hormone (oMt1a-oGH) transgene-carrier lines, one from a high -growth background (TM) and one from a control background (TC), and two non transgenic lines, one from each of these genetic backgrounds (NM and NC, re spectively). A fifth line (CC), not containing the transgene, served as a r andomly selected control. The initial frequency of the oMt1a-oGH transgene construct in the TM and TC lines was 0.5. The frequency of transgenic femal es sampled at generations 7 and 8 of selection was 84.0% and 6.1% in the TC and TM lines, respectively. No significant female infertility differences were detected between transgene-carrier and non-transgenic lines or between transgenic and non-transgenic mice within carrier lines, whereas high-grow th background lines had a higher infertility than control background lines (P < 0.05). Correlated responses in the TC transgene-carrier line were sugg estive of reduced reproductive performance as indicated by increased post-i mplantation mortality (P < 0.05), number of dead fetuses plus implants (P < 0.05), and loss of fetuses from day 16 to parturition (P < 0.001). For the first two traits, the negative correlated responses were accounted for by the reduced performance of transgenic compared with non-transgenic females. Embryos carrying the transgene may also have a lower viability. In contras t, the NC non-transgenic line did not exhibit reduced reproductive performa nce for these traits. The low frequency of the transgene in the high-growth background TM line was associated with reduced fitness and a lower additiv e effect for 8-week body weight compared with the control background TC lin e.