Bacon S J, McClintock M K
Department of Organismal Biology and Anatomy, University of Chicago, IL 60637, USA.
J Reprod Fertil. 1999 Nov;117(2):403-11. doi: 10.1530/jrf.0.1170403.
In rats, dams that conceive in their postpartum oestrus and then lose their firstborn litter bias the sex ratio of the litter toward females in utero. The present study identifies the source of litter sex ratio bias in these postpartum pregnant non-lactating dams. The female bias arises first through the postconception loss of embryos, and second, the loss occurs in midpregnancy between the attachment of the blastocyst to the uterine wall on day 5 and full metrial gland development on day 14. Some pregnancies were restricted to one uterine horn to see if this loss (and thus the opportunity for litter sex ratio biasing) was influenced by local factors operating within the uterine horn. Embryonic loss was more closely associated with the number of embryos implanting in a single horn than with the number implanting in the litter, demonstrating that local crowding within a horn is sufficient for the preferential loss of male embryos. This loss did not cause an obvious decrease in the size of the live-born litter because only those horns with a surfeit of embryos lost them. This process was the same in the right and left horns; both carried and lost the same numbers of embryos. A dam that conceives in her postpartum oestrus and then loses her suckling litter forgoes the implantation delay and uterine healing caused by lactation. Male embryos are less successful at implanting in a uterus only recently vacated by a previous litter.
在大鼠中，在产后发情期受孕然后失去第一窝幼崽的母鼠会使子宫内的幼崽性别比例偏向雌性。本研究确定了这些产后怀孕的非哺乳期母鼠中幼崽性别比例偏差的来源。雌性偏差首先通过受孕后胚胎的丢失出现，其次，这种丢失发生在妊娠中期，即第5天胚泡附着于子宫壁至第14天完全形成蜕膜腺之间。一些妊娠被限制在一个子宫角，以观察这种丢失（以及因此产生的幼崽性别比例偏差的机会）是否受到子宫角内局部因素的影响。胚胎丢失与单个子宫角内植入的胚胎数量比与窝内植入的胚胎数量更密切相关，这表明子宫角内的局部拥挤足以导致雄性胚胎优先丢失。这种丢失并没有导致活产幼崽数量明显减少，因为只有那些胚胎过多的子宫角才会丢失胚胎。左右子宫角的这个过程是相同的；两者携带和丢失的胚胎数量相同。在产后发情期受孕然后失去哺乳幼崽的母鼠会避免因哺乳引起的植入延迟和子宫愈合。雄性胚胎在刚被前一窝幼崽腾出的子宫内植入的成功率较低。
