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

单合子多胚胎是一种无性繁殖形式，通过合子后阶段的分裂产生具有相同性源基因的克隆后代。多胚胎的持续存在让进化生物学家感到困惑，因为它似乎结合了克隆和有性繁殖的不同健康劣势，同时妥协了各自的好处。环口苔藓动物是苔藓动物的一个古老的系统发育目，在晚三叠世开始发育出巨大的孵卵室。在最近的形式中，单独的孵化室容纳多只幼虫。根据19世纪末和20世纪初的显微镜证据，每一窝幼虫都是由一个初级胚胎的迭代萌发而来的。最近，对环口亚种的杂交胚胎和母体群体进行了分子基因分型，确凿地证实了单合子多胚胎存在，并表明每个孵化室内克隆的单一基因来自于通过水媒精子进行的异交。Harmer关于三个亚目的多胚胎的最初组织学推断得到了Borg的广泛工作的证实，证据表明在现存的两个亚目中也有相同的过程。这是合理的推断，扩大的孵化室，记录在所有现存的环口家族中，除了Cinctiporidae，与胚胎发芽有关。然而，在不知道母亲和胚胎的基因组成的情况下，不能排除在育种室内进行多次受精、自体受精或孤雌生殖的可能性。这样的可能性对于理解这种矛盾的生殖模式的保留和确定其在环口动物中可能的适应意义具有重要意义。在这方面同样重要的是，孵卵室的发育是否是由水传播的同种异体精子引起的，就像在唇口的Celleporella hyalina(和其他一些受精母体)中发现的那样，在这种情况下，雌性的投资是由异体精子的摄取触发的。假设1：所有主要环口类群中的个体孵化室孵化出相同基因的非自体、非孤雌生殖胚胎。到目前为止，利用为该物种开发的微卫星，只在齿状克里亚科动物中从遗传上证实了环口动物中的多胚胎。学生将使用分子标记(ISSR)来研究一系列环口物种的遗传组成，这些物种被选为代表所有现存的支系，利用自然历史博物馆正在开发的关于环口动物系统发育的最新了解。在生殖隔离的实验室饲养将测试自交受精的可能性。假设2：环口的孵卵室发育是由同种异体精子的存在触发的。在环口菌落内和环口菌落之间零星分布的孵卵室提出了关于它们发育控制的重要问题。特别是，同种异体精子的摄取是否会触发孵卵室的形成，就像某些唇口动物那样？在菌落生长过程中，同种精子的存在对生殖细胞发育的影响将在实验室培养中阐明，与持续隔离的对照菌落相比。补充调查：通过多个繁殖周期和额外的交配(暴露于同种异体精子直到停止释放幼虫)继续培养群体，将决定胚胎克隆的持续时间和数量范围，以及类淋巴是否可以用于不同基因的后续后代。假设3：环口是否通过接受精子的差异来调节近亲交配？唇口(非多胚胎)苔藓虫透明苔藓虫根据来源和受体之间的亲缘关系而不同地利用精子。同样的模式也出现在多胚胎的环口中吗？不同亲缘关系的蜂群之间的试验性实验室配对将检验这一点。

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.