Delayed Insemination Results in Embryo Mortality in a Brooding Ascidian

Abstract

We explored the effects of temporal variation in sperm availability on fertilization and subsequent larval development in the colonial ascidian Botryllus schlosseri, a brooding hermaphrodite that has a sexual cycle linked to an asexual zooid replacement cycle. We developed a method to quantify the timing of events early in this cycle, and then isolated colonies before the start of the cycle and inseminated them at various times. Colony-wide fertilization levels (assayed by early cleavage) increased from zero to 100% during the period when the siphons of a new generation of zooids were first opening, and remained high for 24 h before slowly declining over the next 48 h. Because embryos are brooded until just before the zooids degenerate at the end of a cycle, delayed fertilization might also affect whether embryos can complete development within the cycle. Consequently, we also determined the effect of delayed insemination on successful embryo development through larval release and metamorphosis. When fertilization was delayed beyond the completion of siphon opening, there was an exponential decline in the percentage of eggs that ultimately produced a metamorphosed larva at the end of the cycle. Thus, even though the majority of oocytes can be fertilized when insemination is delayed for up to 48 h, the resulting embryos cannot complete development before the brooding zooids degenerate.

Spatial distribution of ascidian sperm: two-dimensional patterns and short vs. time-integrated assays

“…The buds expand and assume the function of the older zooids, which are quickly resorbed. Eggs are viable to be fertilized as soon as the siphons of the new generation of zooids open (Stewart-Savage et al 2001), and once fertilized develop into embryos that are released as tadpole larvae at the end of the cycle (Milkman 1967). Sperm release commences ~16 h after the start of a cycle and continues for several days, but does not peak until a few days after siphon opening .…”

“…Sperm release commences ~16 h after the start of a cycle and continues for several days, but does not peak until a few days after siphon opening . Colonies neither selffertilize nor store sperm (Stewart-Savage et al 2001). Fertilization appears to be a very efficient process, with long-lived, dilute sperm (Johnson & Yund 2004) captured throughout a 24 h period of egg viability (Stewart-Savage et al 2001).…”

“…Because our study organism has an extended period of egg viability (ca. 24 h; Stewart-Savage et al 2001), at one site we also compared our short-term assays with time-integrated fertilization levels from colonies deployed in the field to assess the relationship between the instantaneous fertilization values and absolute fertilization levels at different sites.…”

Botryllus schlosseri: A model ascidian for the study of asexual reproduction

“…1B) and discharges most of the sperm 1-2 days after ovulation, thus avoiding selfing (Milkman, 1967;Sabbadin, 1971;Sabbadin and Zaniolo, 1979;Stewart-Savage et al, 2001).…”

Fertilization success in <i>Galeolaria caespitosa</i> (Polychaeta: Serpillidae): gamete characteristics, role of sperm dilution, gamete age, and contact time

“…Although oocytes of the ascidian Ascidia mentula could be fertilized up to 144 hours after spawning, normal larvae were obtained from ova fertilized within 96 hours of spawning (Havenhand, 1991). In another ascidian Botryllus schlosseri (Stewart-Savage et al, 2001) viable larvae are only produced when fertilization occurs within 19 hours although >50% of eggs can be fertilized within 38-49 hours. Similarly, eggs of the polychaete Nereis virens can be fertilized for 96 hours after spawning, but almost all zygotes fertilized 48 hours after spawning develop abnormally (Williams and Bentley, 2002).…”