LB 21-188 - Intraspecific variation can provide a framework to understand life cycle diversification in green algae

Sexual organisms diversify their life cycles, which is a fundamental characteristic that facilitates or constrains adaptive evolution of other phenotypes. In many plants' life cycles, fertilized zygotes develop into diploid multicellular sporophytes. Some plants have a life cycle in which zygotes develop into a unicellular body. There is an unverified hypothesis that different life cycles evolved via adaptations to different environments. In addition to these life cycles, we recently found an intraspecific life cycle variation in which zygotes develop into both sporophytes and unicellular bodies in green algae. Phenotypic variation can evolve and maintain as an optimal strategy to cope with fluctuating environments. The objectives of this study are to explore a principle of life cycle diversification under different environments. To address this, we focused on that the ratio of sporophyte development in the zygote developmental variation determines the type of life cycles and that its optimal ratio may differ under different environments. Additionally, sporophytes are more vulnerable to intertidal environmental pressures than unicellular microscopic cysts. Here we show preliminary results of our mathematical model incorporating fitness parameters in green algae. We found that sporophyte development occurred despite of the volume of zygotes and genotypes, suggesting that this developmental variation is a bet-hedging strategy to cope with temporal environmental fluctuation. Comparisons of offspring numbers between the different developmental fates of zygotes showed potential fitness advantages of sporophyte development. A preliminary mathematical model suggests that the different life cycle types (sporophyte, unicellular cysts, intraspecific variation) can maximize fitness under different environments. In conclusion, this study suggests that life cycle differences among green algae can emerge from adaptations to different environments. Completing this study can provide a theoretical framework to explain the evolution of diversified life cycles in green algae.