Vertical transmission of a nematode from female lizards to the brains of their offspring

Parasites have evolved a diversity of life styles that exploit the biology of their hosts. Some nematodes that parasitize mammals pass via the placenta or milk from one host to another. Similar cases of vertical transmission have never been reported in avian and non-avian reptiles, suggesting that egg laying may constrain themeans of parasite transmission. However, here we report the first incidence of transovarial transmission of a previously undescribed nematode in an egg-laying amniote, the common wall lizard, Podarcis muralis . Nematodes enter the developing brain from the female ovary early in embryonic development. Infected lizard embryos develop normally and hatch with nematodes residing in their braincase. We present a morphological and molecular phylogenetic characterization of the nematode and suggest that particular features of lizard biology that are absent from birds and turtles facilitated the evolutionary origin of this novel life history.寄生虫已演化出丰富多样的生活史策略，以利用宿主的生物学特性。部分寄生于哺乳动物的线虫可通过胎盘或乳汁在宿主间垂直传播。鸟类与非鸟类爬行动物中从未有过此类垂直传播的报道，这提示产卵行为可能限制了寄生虫的传播途径。然而，本研究首次报道了产卵羊膜动物——普通壁蜥（Podarcis muralis）中一种此前未被描述的线虫的经卵传播（transovarial transmission）事件。该线虫可在胚胎发育早期从雌性宿主卵巢侵入发育中的脑组织。受感染的蜥蜴胚胎可正常发育并孵化，其颅腔内仍留存有线虫。本研究对该线虫进行了形态学与分子系统发育特征分析，并提出：鸟类与龟类所不具备的部分蜥蜴生物学特性，或许推动了这一新奇生活史的演化起源。