DEVELOPMENT
3486bodies undergo apoptosis and are removed via phagocytic cells in the blood, the primary bud migrates into the newly vacated region of the colony, opening its siphons and becoming a zooid, the secondary bud becomes the primary bud, and a new secondary bud begins to develop. Thus, the life history of Botryllus consists of a constant succession of individual zooids, each with a three-week lifespan (Lauzon et al., 2002).Asexual development (blastogenesis) takes 14 days under laboratory conditions, and can be divided into seven distinct visual stages ( Fig. 1; stages A-1 through D) (Lauzon et al., 2002). A new generation starts as a secondary bud, first visible as a thickening of the peribranchial epithelium of a primary bud ( Fig. 1; stage A1), which evaginates and forms a closed vesicle ( Fig. 1; stage A-2 through B-2). Next, a series of epithelial invaginations and protrusions ( Fig. 1; stage C-1) differentiate into somatic tissues and organs ( Fig. 1; stage C-2). After seven days ( Fig. 1; stage D), the secondary bud transitions to a primary bud and continues to develop. At day 14, the siphons open and the primary bud becomes a filterfeeding adult zooid. Each zooid can generate multiple buds each week, so the colony will eventually expand asexually. While interconnected, the zooids and buds develop independently, and individuals can be separated from the colony without disturbing their growth (i.e. subcloning), thus multiple experiments can be done on a single genotype.Following metamorphosis, colonies undergo at least 8-12 developmental cycles prior to the first appearance of gametes (sexual maturity). In addition, populations show seasonal fertility, and in the lab cycle in and out of reproductive (fertile) and nonreproductive (infertile) states. However when the colony is fertile development of the gametes is synchronized with somatic development (Mukai, 1977;Mukai and Watanabe, 1976;Sabbadin and Zaniolo, 1979). The first appearance of gonads occurs in the secondary bud (stage B), when mobile progenitors in the blood migrate to a region between the inner epithelium and the epidermis and begin to proliferate. Concurrently, oocytes at various stages of development also appear (Fig. 1). Over the next 10 days, the medial region of the blastema will differentiate into the lobular testis, while the lateral region will become the ovary (Sabbadin and Zaniolo, 1979). For the latter, one or several oocytes will become fixed on the epithelia of the peribranchial chamber, and an oviduct will form from the outer follicular layer. Upon transition to the adult zooid, mature eggs will immediately ovulate into the peribranchial chamber, be fertilized by exogenous sperm, and develop in situ. Several hours to days later the testes will complete development, and sperm will be released into the peribranchial chamber and flushed into the water column, fertilizing neighboring colonies (Johnson and Yund, 2004). The time lag between ovulation and sperm release (protogyny) prevents self-fertilization.Given this plastici...
