Uterine molecular changes for non‐invasive embryonic attachment in the marsupials Macropus eugenii (Macropodidae) and Trichosurus vulpecula (Phalangeridae)

Abstract

Pregnancy in mammals requires remodeling of the uterus to become receptive to the implanting embryo. Remarkably similar morphological changes to the uterine epithelium occur in both eutherian and marsupial mammals, irrespective of placental type. Nevertheless, molecular differences in uterine remodeling indicate that the marsupial uterus employs maternal defences, including molecular reinforcement of the uterine epithelium, to regulate embryonic invasion. Non-invasive (epitheliochorial) embryonic attachment in marsupials likely evolved secondarily from invasive attachment, so uterine defences in these species may prevent embryonic invasion. We tested this hypothesis by identifying localization patterns of Talin, a key basal anchoring molecule, in the uterine epithelium during pregnancy in the tammar wallaby (Macropus eugenii; Macropodidae) and the brush tail possum (Trichosurus vulpecula; Phalangeridae). Embryonic attachment is non-invasive in both species, yet Talin undergoes a clear distributional change during pregnancy in M. eugenii, including recruitment to the base of the uterine epithelium just before attachment, that closely resembles that of invasive implantation in the marsupial species Sminthopsis crassicaudata. Basal localization occurs throughout pregnancy in T. vulpecula, although, as for M. eugenii, this pattern is most specific prior to attachment. Such molecular reinforcement of the uterine epithelium for non-invasive embryonic attachment in marsupials supports the hypothesis that less-invasive and non-invasive embryonic attachment in marsupials may have evolved via accrual of maternal defences. Recruitment of basal molecules, including Talin, to the uterine epithelium may have played a key role in this transition.

Non‐invasive placentation in the marsupials Macropus eugenii (Macropodidae) and Trichosurus vulpecula (Phalangeridae) involves redistribution of uterine Desmoglein‐2

“…Given that uterine Desmoglein‐2 redistribution is relatively conserved across mammalian groups, and occurs irrespective of placental type, the specific pattern of localization may be an important uterine strategy involved in facilitating placentation, rather than as a response to increase maternal control over resource allocation, and may play a role in initial embryonic implantation. In marsupials, including M. eugenii and T. vulpecula , reduced cell–cell adhesion resulting from Desmoglein‐2 redistribution may be compensated for by molecular reinforcement of the basal plasma membrane of the uterine epithelium prior to implantation (Laird, Dargan, et al, ). Hence, the molecular patterns of Desmoglein‐2 in the lateral plasma membrane and Talin in the basal plasma membrane may play complementary roles during pregnancy in M. eugenii and T. vulpecula , resulting in facilitation and restriction of embryonic invasion, respectively, and thus enabling successful placentation (Poon, Madawala, Dowland, & Murphy, ).…”

“…Since marsupials appear to lack mechanisms of regulating embryonic invasion in the uterine stroma, the uterine epithelium likely plays a more important role in regulating implantation in marsupials than in eutherian mammals. This tenet is supported by molecular reinforcement of focal adhesions—basal connections between the uterine epithelium and the underlying stromal cells—prior to implantation in marsupials, irrespective of placentation type, thus strengthening the uterine epithelium as a barrier to embryonic invasion (Fowden & Moore, ; Laird, Turancova, McAllan, Murphy, & Thompson, ; Laird, Dargan, et al, ; Laird, Turancova, McAllan, Murphy, & Thompson, ). In contrast, basal adhesion of the uterine epithelium is lost during this same period in eutherian mammals as focal adhesions disassemble, thus facilitating embryonic invasion (Kaneko, Lindsay, & Murphy, ; Kaneko, Day, & Murphy, ; Murphy, ).…”

“…We addressed this by identifying Desmoglein‐2 localization in the uterus throughout pregnancy in the marsupial species Macropus eugenii (Macropodidae) and Trichosurus vulpecula (Phalangeridae) (Figure b) (Freyer, Zeller, & Renfree, ; Pilton & Sharman, ). Both species have non‐invasive placentation and undergo basal reinforcement of the uterine epithelium leading to implantation (Laird, Dargan, et al, ). As non‐invasive placentation likely evolved independently in macropodids and phalangerids (Mess & Ferner, ), comparison of the placental features of M. eugenii and T. vulpecula can identify the shared, essential mechanisms of non‐invasive placentation in marsupials.…”

“…M. eugenii has a predictable annual breeding cycle (Renfree & Shaw, ; Tyndale‐Biscoe & Renfree, ; see Laird, Hearn, Shaw, & Renfree []; and Laird, Dargan, et al [] for summary timelines). Mating occurs during a post‐partum oestrus (Renfree, ; Rudd, ; Tyndale‐Biscoe & Renfree, ), and ovulation of a single egg occurs the following day, with ovulation alternating between ovaries (monovular).…”

“…The brushtail possum T. vulpecula has a 28‐day oestrous cycle (Pilton & Sharman, ; Tyndale‐Biscoe, ) and a 17.5‐day gestation period (Pilton & Sharman, ; Sizemore, Hurst, & McLeod, ). Like M. eugenii , T. vulpecula is monovular, with ovulation occurring 1–2 days after oestrus (see Laird, Dargan, et al, ; Laird, McShea, McAllan, Murphy, & Thompson, for summary timelines); however, embryos of T. vulpecula do not undergo developmental arrest. The embryo attaches non‐invasively approximately 14 days after conception, with birth 3–4 day later (Pilton & Sharman, ; Tyndale‐Biscoe, ).…”