Weedall Robert T, Robinson Michael, Smith Judith E, Dunn Alison M
Faculty of Biological Sciences, Institute for Integrative and Comparative Biology, University of Leeds, Miall Building, Clarendon Way, Leeds LS2 9JT, UK.
Int J Parasitol. 2006 Jun;36(7):749-56. doi: 10.1016/j.ijpara.2006.02.020. Epub 2006 Mar 29.
Feminising microsporidian parasites are transmitted vertically from generation to generation of their crustacean hosts. Little is known about the mechanisms underpinning vertical transmission, in particular, parasite transmission to the host gonad during host development. Here, we investigate the burden and distribution of two species of vertically transmitted, feminising microsporidia (Dictyocoela duebenum and Nosema granulosis) during early embryogenesis (zygote to eight-cells) of the Gammarus duebeni host. Parasite burden differs between the two parasites with N. granulosis being higher by a factor of 10. Whilst D. duebenum replicates during the first few host cell divisions, there is no increase in N. granulosis burden. Only merogonic parasite stages were observed in the host embryo. Distribution of both parasites was non-random from the two-cell embryo stage, indicating biased parasite segregation at host cell division. Dictyocoela duebenum burden was low in the germline and somatic gonad progenitor cells but was highest in the ectoderm precursors, leading us to propose that the parasite targets these cells and then secondarily infects the gonad later in host development. Targeting by N. granulosis was less specific although there was a persistent bias in parasite distribution throughout host cell divisions. Parasite burden was highest in the ectoderm precursors as well as the germline progenitors leading us to suggest that, in addition to using the ectodermal route, N. granulosis may also target germline directly. Biased segregation will be adaptive for these parasites as it is likely to lead to efficient transmission and feminisation whilst minimising virulence in the host.
使宿主雌性化的微孢子虫寄生虫在其甲壳类宿主的代际间垂直传播。关于垂直传播的潜在机制，尤其是寄生虫在宿主发育过程中向宿主性腺的传播，我们知之甚少。在此，我们研究了两种垂直传播的、使宿主雌性化的微孢子虫（杜氏网孢虫和颗粒 Nosema）在杜氏钩虾宿主早期胚胎发育（合子到八细胞期）期间的负荷和分布情况。两种寄生虫的寄生虫负荷有所不同，颗粒 Nosema 的负荷高出 10 倍。虽然杜氏网孢虫在宿主细胞最初几次分裂期间进行复制，但颗粒 Nosema 的负荷并未增加。在宿主胚胎中仅观察到裂殖生殖阶段的寄生虫。从二细胞胚胎期开始，两种寄生虫的分布均非随机，这表明在宿主细胞分裂时寄生虫存在偏向性分离。杜氏网孢虫在生殖系和体细胞性腺祖细胞中的负荷较低，但在外胚层前体细胞中最高，这使我们推测该寄生虫以这些细胞为目标，然后在宿主发育后期继发感染性腺。颗粒 Nosema 的靶向性较低，尽管在整个宿主细胞分裂过程中寄生虫分布存在持续的偏向性。寄生虫负荷在外胚层前体细胞和生殖系祖细胞中最高，这使我们认为，除了通过外胚层途径，颗粒 Nosema 也可能直接靶向生殖系。偏向性分离对这些寄生虫具有适应性，因为这可能导致高效传播和雌性化，同时将对宿主的毒力降至最低。
