Carpenter Meredith A, Powell John H, Ishizuka Katherine J, Palmeri Karla J, Rendulic Snjezana, De Tomaso Anthony W
Department of Earth Sciences, Stanford University, Stanford, CA 94306, USA.
Biol Bull. 2011 Feb;220(1):57-70. doi: 10.1086/BBLv220n1p57.
The colonial ascidian Botryllus schlosseri undergoes a histocompatibility reaction that can result in vascular fusion of distinct genotypes, creating a chimera. Chimerism has both potential benefits, such as an immediate increase in size that may enhance growth rates, and costs. For the latter, the presence of multiple genotypes in a chimera can lead to competition between genetically distinct stem cell lineages, resulting in complete replacement of somatic and germline tissues by a single genotype. Although fusion can occur at any point after metamorphosis, previous studies have focused on chimeras created from sexually mature adults, where no benefit to chimerism has been documented. Here we focus on the costs and benefits of fusion between juveniles, characterizing growth rates and patterns of somatic and germline chimerism after natural and controlled fusion events. We also compared outcomes between low- and high-density growth conditions, the latter more likely representative of what occurs in natural populations. We found that growth rates were density-dependent, and that only chimeras grew under high-density conditions. We also observed a positional component to a post-fusion event called resorption, indicating that extrinsic factors were important in this process. Patterns of germline and somatic chimerism and dominance in chimeras made from fused juveniles were equivalent to those after fusion of sexually mature adults, and there were no age-related differences in these processes. Finally, by using genetic markers that could retrospectively assign genotypes, we also found that the majority of individual testes in a chimera were clonally derived.
群居海鞘Botryllus schlosseri会发生组织相容性反应，这可能导致不同基因型的血管融合，形成嵌合体。嵌合体既有潜在益处，比如体型立即增大可能提高生长速率，也有代价。就代价而言，嵌合体中多种基因型的存在会导致基因不同的干细胞谱系之间产生竞争，结果是单一基因型完全取代体细胞和生殖细胞组织。虽然融合可在变态后的任何时间发生，但以往研究聚焦于由性成熟成体形成的嵌合体，在这类嵌合体中尚未有关于嵌合有益的记录。在此，我们关注幼体之间融合的代价和益处，描述自然和受控融合事件后体细胞和生殖细胞嵌合的生长速率及模式。我们还比较了低密度和高密度生长条件下的结果，后者更可能代表自然种群中的情况。我们发现生长速率取决于密度，且只有嵌合体在高密度条件下生长。我们还观察到一种融合后事件（称为吸收）存在位置因素，这表明外在因素在该过程中很重要。由融合幼体形成的嵌合体中生殖细胞和体细胞的嵌合模式及优势与性成熟成体融合后的情况相同，且在这些过程中不存在与年龄相关的差异。最后，通过使用能够追溯确定基因型的遗传标记，我们还发现嵌合体中大多数单个睾丸是克隆衍生的。
