Stewart-Savage J, Wagstaff B J, Yund P O
Department of Biological Sciences, University of New Orleans, New Orleans, Louisiana 70148.
Biol Bull. 1999 Feb;196(1):63-9. doi: 10.2307/1543168.
Colonies of the ascidian Botryllus schlosseri (a cyclical hermaphrodite) exhibit extreme variability in egg production, and there is a large genetic component to this phenotypic variation. Therefore, the developmental bases of variation among different genotypes was investigated. Colonies differing in egg production (assayed as number of eggs per asexual bud) were cultured in a common garden experiment, and buds were collected and fixed early in the reproductive cycle. The buds were serially sectioned, and the number and size of the oocytes in the developing ovaries were determined for the different genotypes. Because the buds were collected prior to the onset of vitellogenesis, they contained oocytes at the three previtellogenic stages. In reproductive colonies (>0.7 eggs per bud), there were negative relationships between the final number of eggs per bud and (1) the total number of oocytes present, (2) the number of stage 1 oocytes present, and (3) the number of stage 2 oocytes present. There was no relationship between these parameters in nonreproductive colonies (<0.3 eggs per bud). In contrast, the number of stage 3 oocytes per bud was positively correlated with the final number of eggs per bud in both reproductive and nonreproductive colonies. In reproductive animals there was a negative relationship between the total number of oocytes per bud and the percentage of oocytes at stage 3 in oogenesis. A principal component analysis revealed that a single vector equally weighted for the number of eggs per bud, the total number of oocytes per bud, and the percentage of oocytes at stage 3 accounted for 84% of the observed variation in reproductive colonies. These data indicate that the phenotypic variation in egg production among the B. schlosseri colonies in the Damariscotta River, Maine, is controlled by genetic variation in both the number of oocytes that populate developing ovaries, and the percentage of oocytes that reach stage 3 in oogenesis.
海鞘Botryllus schlosseri（一种周期性雌雄同体动物）的群体在产卵量上表现出极大的变异性，并且这种表型变异有很大的遗传成分。因此，研究了不同基因型之间变异的发育基础。在一个共同花园实验中培养了产卵量不同（以每个无性芽中的卵数来测定）的群体，并在生殖周期早期收集芽并固定。对芽进行连续切片，然后确定不同基因型发育卵巢中卵母细胞的数量和大小。由于芽是在卵黄发生开始之前收集的，所以它们含有处于三个卵黄发生前阶段的卵母细胞。在生殖群体（每个芽>0.7个卵）中，每个芽的最终卵数与（1）存在的卵母细胞总数、（2）存在的1期卵母细胞数以及（3）存在的2期卵母细胞数之间呈负相关。在非生殖群体（每个芽<0.3个卵）中，这些参数之间没有关系。相反，在生殖和非生殖群体中，每个芽的3期卵母细胞数与每个芽的最终卵数呈正相关。在生殖动物中，每个芽的卵母细胞总数与卵母细胞在卵子发生过程中处于3期的百分比之间呈负相关。主成分分析表明，一个对每个芽的卵数、每个芽的卵母细胞总数以及3期卵母细胞百分比进行同等加权的单一向量占生殖群体中观察到变异的84%。这些数据表明，缅因州达马里斯科塔河的Botryllus schlosseri群体在产卵量上的表型变异是由发育卵巢中卵母细胞数量的遗传变异以及卵子发生过程中达到3期的卵母细胞百分比所控制的。
