Figure 1

Botryllus schlosseri sexual and asexual development (blastogenesis) and anatomy. A) Sexual cycle showing the larva stage and metamorphosis into oozoid. B) Asexual cycle – blastogenesis: Dorsal views of zooids (green frame), primary buds (yellow frame) and secondary buds (red frame). A secondary bud appears as a thickening of the epidermis and the peribranchial chamber leaflet of the primary bud (stage A1), which evaginates into a closed vesicle (stage B2), followed by organogenesis (stages C1-D). Gonadogenesis occurs in the secondary bud from mobile precursors (blue; stages B1-C2). During takeover (stages C2-D), the secondary bud becomes the primary bud and a new blastogenic cycle begins for the next secondary bud. After the second takeover event, the primary bud opens its siphons and becomes a functional adult (zooid). In fertile colonies (as illustrated here), the hermaphrodite gonad fully matures on both sides of the zooid. C) Schematic showing the anatomy of an adult fertile zooid with two primary buds. D and E) Ventral live image of both infertile and a fertile colonies (respectively). Arrows point to testes and arrowheads point to eggs. Panels A and B are modified from Brown et al. 2009, Laird et al. 2005 and Laird and De Tomaso 2005.