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

环口苔藓虫的多胚、育雏室启动和精子利用。

• 批准号: NE/G012644/1
• 金额: $ 8.64万
• 依托单位: Bangor University
• 依托单位国家: 英国
• 项目类别: Training Grant
• 财政年份: 2009
• 资助国家: 英国
• 起止时间: 2009 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FG012644%2F1
• 关键词: Polyembryony; brood; chamber; initiation; sperm

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.

单合子多胚胎是一种无性生殖形式，通过合子后阶段的分裂产生具有相同性衍生基因型的无性繁殖。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生殖的相对适合性缺点，同时损害了各自的好处。环口虫是苔藓虫的一个古老的系统发育目，在晚三叠世开始发育体积庞大的育雏室。在最近的形式中，单个育雏室容纳多个幼虫。根据19世纪末和20世纪初显微镜的证据，每一窝都是由初级胚胎的反复出芽而产生的。最近，对一种环纹虫（Crisia denticulata）的育胚和母系菌落的分子基因分型证实了单合子多胚现象，并表明在每个育室内克隆的单一基因型是通过水媒精子异种杂交产生的。哈默在三个亚目中有多胚胎的原始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同样的过程的证据。我们有理由推断，除环口科外，所有现存环口科都有增大的育苗室，这与胚胎出芽有关。然而，在不了解母体和胚胎的基因型组成的情况下，不能排除在育雏室内多次受精、自受精或孤雌生殖的可能性。这种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性意义具有重要意义。在这方面同样重要的是，育室的发育是否由水生同种异体精子的存在诱导，就像在透明小玻璃虫（和其他一些精胚物质）中发现的那样，雌性的投入是由同种异体精子的吸收引发的。假设1：在所有主要的环口动物群体中，单个的育雏室都有相同基因型的非自交、非孤雌生殖的胚胎。迄今为止，环口动物的多胚胎现象只在齿状Crisia denticulata中得到了遗传学上的证实，使用的是为该物种开发的微卫星。该项目将利用美国自然历史博物馆（Natural History Museum）正在开发的最新环孔虫系统发育研究成果，利用分子标记（ISSRs）技术，研究一系列环孔虫物种的遗传组成，这些物种代表了所有现存的进化支。隔离繁殖的实验室饲养将检验自交受精的可能性。假设2：卵室的发育是由同种异体的存在引起的。巢室的零星分布内部和之间的回旋石群体提出了重要的问题，控制其发展。特别是，异体精子的摄取是否会触发育室的形成，就像在一些切口动物中一样？在实验室培养中，与持续分离的对照菌落相比，同种精子的存在对菌落生长过程中性腺发育的影响将得到阐明。补充调查：通过多个生殖周期和额外的交配（暴露于异体精子直到停止释放幼虫）继续培养菌落将决定胚胎克隆的持续时间和数量范围，以及是否可以将淋虫用于不同基因型的连续后代。假设3：环口是否通过对精子的不同接受来调节近亲繁殖？舌口（非多胚胎）苔藓虫透明细胞利用精子的差异取决于来源和受体之间的关系。同样的模式是否发生在多胚胎环口？不同亲缘关系的群体之间的试验性实验室交配将检验这一点。