Risk and the Evolution of Cell-Cycle Durations of Embryos

Abstract

Abstract. Embryos at low risk evolve slower development rates. In seven independent evolutionary contrasts for marine invertebrates (two in asteroids, three in gastropods, one each in phoronids and brachiopods) the more protected embryos had longer cell cycles from first to second cleavage than less protected planktonic embryos. Protected embryos had longer cell cycles even when protected eggs were smaller than planktonic eggs. In an eighth contrast, among tunicates, the embryonic cell cycle was unrelated to brooding and nearly proportional to egg size, but the literature provides examples of especially slow development in some brooding tunicates. The faster development of planktonic embryos is consistent with published estimates of greater mortality rates for planktonic larvae than for embryos in broods or egg masses. Examples from the literature for annelids, arthropods, holothuroids, and chordates also demonstrated longer embryonic cell cycles for more protected embryos with no consistent effect of egg size on cell-cycle duration. Longer cell cycles presumably reduce the benefits of protecting offspring because of longer exposure to whatever hazards remain, but slow development may permit compensating benefits. Hypothesized benefits of longer cell cycles include less maternal investment in rate-limiting materials, more or different transcription, and correction of errors. Such trade-offs are independent of feeding and growth and are influenced by parental protection.

Effects of evolution on egg development time

“…The time embryos require to pass from the 2 cell to the 4 cell phase (a cell cycle time) was measured by Strathmann et al (2002) for a range of aquatic organisms (Echinodermata,33 Egg mass (µg C) All data corrected to 15°C using average activation energy value of 0.65 (Gillooly et al 2002), and divided into egg disposition category described in Mollusca, Phoronida, Brachiopoda and Chordata). They observed that embryos at low risk had slower development rates (measured as the time from first to second cleavage of the egg).…”

“…They observed that embryos at low risk had slower development rates (measured as the time from first to second cleavage of the egg). Strathmann et al (2002) also considered aggregated and brooded eggs to be less vulnerable. In Fig.…”

“…Nonetheless, we suggest that protection strategies are important and impact egg hatch times between environment types, and not simply within the marine environment. Strathmann et al (2002) compared time to first locomotion in embryos of asteroids, gastropods, phoronids, and brachiopods, and found that unprotected embryos initiated locomotion at both an earlier stage and younger age than protected benthic embryos. Furthermore, structures in the first swimming stage are generally less developed in pelagic spawned eggs than in eggs that are brooded or deposited in egg masses (Strathmann 1987, Kiørboe & Sabatini 1994, Satoh 1994.…”

“…Furthermore, structures in the first swimming stage are generally less developed in pelagic spawned eggs than in eggs that are brooded or deposited in egg masses (Strathmann 1987, Kiørboe & Sabatini 1994, Satoh 1994. As pointed out by Strathmann et al (2002), time to hatching combines rate of development and stage at hatching in a single measurement. Hatchlings in different taxa can be in radically different states of development.…”

“…Thus, different rates of development (Fig. 3) Strathmann et al (2002) comprising data from taxa Echinodermata, Mollusca, Phoronida, Brachiopoda and Chordata. All data corrected to 15°C using average activation energy value of 0.65 (Gillooly et al 2002).…”