Reproduction in field populations of the five species and 13 of 14 subspecies of native Australian Rattus is compared, using both materials personally collected and museum specimens. We have separated each subspecies into juvenile and adult categories, the dividing point reflecting the approximate size at which sexual maturity is reached. Occurrence of pregnancies and juveniles, treated monthly, permit estimates of time and length of seasonal breeding for each subspecies. Information on gonads and accessory sex glands is given for both breeding and non-breeding adults and for juveniles. All Australian subspecies of Rattus have a fundamentally similar mechanism of reproduction. Those attaining high densities have the highest ovulation rates, the largest litter sizes, and the greatest number of teats. Furthermore, in the two subspecies of high reproductive potential for which we have evidence, females can be precocious in attaining sexual maturity under field conditions.
The mosaic-tailed rat, M. cervinipes, is usually an abundant murid where it occurs in the northern regions of Australia. It is an inhabitant of the forests and their fringe associations and is semi-arboreal. Although the annual reproductive performance is unknown, breeding occurs at least in late spring and summer in more southern areas of its distribution and extends into autumn and winter farther north. Litter size ranges from two to four. The nipple-clinging behaviour of the young appears to be of selective advantage as an escape mechanism. Description of the histology of the gonads and accessory sex glands is given for juveniles, breeding adults, and sexually quiescent adults. M. cervinipes is polyoestrous and a spontaneous ovulator and may experience a post-partum oestrus. Melomys is not known to occur in plague proportions in its native habitat; its mode of reproduction favours a prolonged and modest contribution of young within a breeding period.
In most marsupials, placentation involves only the yolk sac; however, in the bandicoot family, Peramelidae, a functional chorioallantoic placentation develops in addition (Hill, 1895, 1897, 1900; Flynn, '22, '23). This duality is viewed as having evolutionary significance because most eutheria have both placentae. Furthermore, the bandicoot trophoblast was reported to vanish from the chorioallantoic site in late gestation (Hill, 1897; Flynn, '23); whereas, the eutherian trophoblast is identifiable throughout later pregnancy and may act as an immunological barrier between maternal and fetal genotypes (Kirby '68). Thus we have re-examined this singular chorioallantoic placenta of the bandicoot in plastic sections with light and electron microscopy. A distinctive feature of the bandicoot placentation is the transformation of the uterine simple columnar luminal epithelium into a highly vascular lining composed almost entirely of discrete syncytial masses (homokaryons). Endometrial blood vessels penetrate among the homokaryons to create a rich network of large diameter capillaries at extremely superficial locations near the maternal surface. In the chorioallantoic placenta (7 mm to 10-11 mm crown-rump embryos) the microvillous surface of the maternal homokaryons interdigitates with the microvillous border of the fetal trophoblast with desmosomal interaction. This trophoblast consists of a single layer of tall columnar undifferentiated cells rich in ribosomes-polysomes, poor in cytoplasmic membranes, and with large nuclei that have distinct clumps of heterochromatin and conspicuous nucleoli. It is thus remarkable that these undifferentiated cells disappear as a recognizable layer later in gestation (12 mm crown-rump embryos). Flynn's hypothesis that the trophoblastic cells disappear by fusing with maternal syncytia gains support from the existence of two populations of nuclei in the syncytial masses only at the chorioallantoic site. One population is comparable to that occurring in the homokaryons pf the yolk sac placenta, i.e., pale staining nuclei with little heterochromatin and small peripheral nucleoli. However, the other nuclei resemble those of the trophoblast cells. Since the trophoblastic cells before their disappearance as a layer possess properties associated with potential for further differentiation, the possibility of fusion between the maternal homokaryons and fetal trophoblastic cells to form heterokaryons composed of two genotypes merits fur
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