Heteromorphic and isomorphic alternations of generations in macroalgae, as adaptation to a seasonal environment.

Macroalgea (seaweeds) show diverse life-cycles. Especially notable is the difference between heteromorphic cycle and isomorphic cycle. Species with heteromorphic life cycle have a large multicellular body in one generation but have a microscopic body in the other generation of a year. This alternation of life forms is linked with the alternation of the genome: one generation is diploid (2n), and the other is haploid (n). In contrast, isomorphic species have both diploid and haploid life forms with very similar morphology, having more than two generations in a year. Here we examine a hypothesis that the diversity of life cycle observed among macroalgae might be adaptations to seasonally changing environments. We develop mathematical models for the optimal life-cycle schedules in the environment where mortality changes seasonally. Then we discuss the condition in which hetermorphic or isomorphic species are more advantageous than the other by population dynamics. We first discuss the optimal reproductive schedule for a heteromorphic species that achieves the maximum population growth rate. It is to have the generation with a large-sized body in the low-mortality season and the generation with a small-sized body in the high-mortality season. In the optimal reproductive schedule of isomorphic species, algae of different generations mature when the body size reaches a common value, but the length of a generation greatly varies between seasons. In a strongly seasonal environment, the heteromorphic species is more profitable than isomorphic species, which is consistent with the observed latitudinal distribution. In a high-mortality and low-mortality environment, the heteromorphic species would be more profitable than isomorphic species, and in a middle-mortality environment, the isomorphic species would be more profitable than heteromorphic species.