Polyembryony, brood-chamber initiation and sperm utilization in cyclostome bryozoans.

单卵多胚是无性繁殖的一种形式，通过合子后阶段的分裂产生具有相同性衍生基因型的克隆窝。多胚现象的持续存在使进化生物学家感到困惑，因为它似乎将克隆和有性生殖的相对不利的适应性结合了联合收割机，同时又牺牲了各自的好处。圆口目是苔藓动物中一个生殖发育古老的目，在晚三叠世开始发育出体积庞大的育巢室。在最近的形式，个别育雏室容纳多个幼虫。根据19世纪末和世纪初显微镜的证据，每窝起源于初级胚胎的迭代出芽。最近，在一个圆口动物物种，Crisia denticulata，育雏胚胎和母体殖民地的分子基因分型，已最终确认单卵多胚性，并表明，在每个育雏室克隆的单一基因型产生通过水生精子的异交。Harmer最初关于三个亚目的多胚现象的组织学推断被博格的大量工作所证实，在剩下的两个亚目中也有相同的过程。这是合理的推断，扩大的育雏室，记录在所有生活的家庭圆口动物，除了artiporidae，是与胚胎出芽。然而，在不知道母体和胚胎的基因型组成的情况下，不能排除在育室内多次受精、自花受精或孤雌生殖的可能性。这种可能性是重要的，在理解保留这种可以说是矛盾的生殖模式，并确定其可能的适应性意义圆口动物。在这方面也很重要的是，无论是育雏室的发展是由水传播的异精子的存在下，已被发现在cheilostome Celleporella hyalina（和其他一些spermcast maters），其中女性投资是由异精子吸收触发。假设1：在所有主要群体的圆口动物的个别育雏室育雏非自交，非单性生殖胚胎相同的基因型。迄今为止，圆口目的多胚现象仅在Crisia denticulata中得到遗传学证实，使用为该物种开发的微卫星。该奖学金将采用分子标记（ISSRs）来研究一系列圆口线虫物种的遗传组成，这些物种被选择来代表所有现存的分支，利用自然历史博物馆正在开发的圆口线虫的最新了解。在生殖隔离的实验室饲养将测试自花受精的可能性。假设2：圆口动物的育室发育是由异精子的存在触发的。零星分布的育雏室内和之间的圆口虫殖民地提出了重要的问题，他们的发展控制。特别是，吸收异体精子是否会触发孵化室的形成，就像一些唇口动物一样？将在实验室培养物中阐明在菌落生长期间同种精子的存在对生殖体发育的影响，并与继续分离的对照菌落进行比较。补充调查：通过多个生殖周期与额外的交配（暴露于异精子通过停止幼虫释放）的殖民地的继续培养将决定胚胎克隆的持续时间和数量的程度，以及生殖子是否可以用于不同基因型的连续育雏。假设3：圆口动物是否通过对精子的不同接受来调节近亲繁殖|?唇口（非多胚）苔藓虫Celleporella hyalina利用精子差异取决于源和受体之间的相关性。在多胚圆口动物中是否也有同样的模式？不同亲缘关系的群体之间的实验室交配试验将检验这一点。

Monozygotic polyembryony is a form of asexual reproduction that proceeds by division of post-zygotic stages to produce clonal broods sharing the same sexually derived genotype. The persistence of polyembryony has puzzled evolutionary biologists because it seems to combine the contrasted fitness disadvantages of cloning and sexual reproduction while compromising the respective benefits. Cyclostomata is a phylogenetically ancient order of bryozoans, which in the Late Triassic began to develop voluminous brood chambers. In Recent forms, individual brood chambers accommodate multiple larvae. Based on the evidence of late 19th and early 20th century microscopy, each brood originates by iterative budding of a primary embryo. Recently, molecular genotyping of brooded embryos and maternal colonies in one cyclostome species, Crisia denticulata, has conclusively confirmed monozygotic polyembryony and indicated that the single genotype cloned within each brood chamber arises from outcrossing via water-borne sperm. Harmer's original histological inference of polyembryony in three suborders was corroborated by the extensive work of Borg, with evidence of the same process in the two remaining extant suborders. It is reasonable to infer that the enlarged brood chamber, recorded in all living families of cyclostomes except the Cinctiporidae, is associated with embryonic budding. Nevertheless, the possibility of multiple fertilizations within the brood chamber, or of self-fertilization, or of parthenogenesis cannot be dismissed without knowing the genotypic composition of mother and embryos. Such possibilities are significant in understanding the retention of this arguably paradoxical reproductive mode and identifying its possible adaptive significance in cyclostomes. Also significant in this respect is whether brood chamber development is induced by the presence of water-borne allosperm, as has been found in the cheilostome Celleporella hyalina (and some other spermcast maters), where female investment is triggered by allosperm uptake. Hypothesis 1: Individual brood chambers in all major groups of cyclostomes brood non-selfed, non-parthenogenetic embryos of identical genotype. To date, polyembryony in cyclostomes has only been proven genetically in Crisia denticulata, using microsatellites developed for that species. The studentship will employ molecular markers (ISSRs) to investigate the genetic composition of broods in a range of cyclostome species chosen to represent all extant clades, utilizing the latest understanding of cyclostome phylogeny being developed at the Natural History Museum. Laboratory rearing in reproductive isolation will test the possibility of self-fertilization. Hypothesis 2: Brood chamber development in cyclostomes is triggered by the presence of allosperm. The sporadic distribution of brood chambers within and between cyclostome colonies raises important questions about the control of their development. In particular, does the uptake of allosperm trigger brood chambers to form, as in some cheilostomes? The effect of the presence of conspecific sperm upon gonozooid development during colony growth will be elucidated in laboratory cultures, compared with control colonies in continuing isolation. Supplementary investigation: Onward culture of colonies through multiple reproductive cycles with additional matings (exposure to allosperm through to cessation of larval release) will determine duration and numerical extent of embryonic cloning, and whether gonozooids can be used for successive broods of different genotype. Hypothesis 3: Do cyclostomes regulate inbreeding by differential acceptance of sperm|? The cheilostome (non-polyembryonic) bryozoan Celleporella hyalina utilizes sperm differentially depending on the relatedness between source and recipient. Does the same pattern occur in the polyembryonic cyclostomes? Trial laboratory matings between colonies of varying relatedness will test this.