Abstract
BackgroundConditional differentiation is one of the most fundamental drivers of biodiversity. Competitive entities (usually species) differ in environmental or ecological niche enabling them to co-exist. Conditional differentiation of haploid and diploid generations is considered to be a requirement for the evolutionary stability of isomorphic biphasic life-cycles and the cause for the natural occurrence of both phases at uneven abundances. Theoretically, stage dependent survival rates are the most efficient way to explain conditional differentiation.ResultsWe tested for conditional differentiation in survival rates among life stages (haploid males, haploid females, and diploids) of Gracilaria chilensis, an intertidal red alga occurring along the Chilean shores. Therefore, the fate of individuals was followed periodically for 3 years in five intertidal pools and, for the first time in isomorphic red algae, a composite model of the instantaneous survival rates was applied. The results showed the survival dependency on density (both competition and Allee effects), fertility, age, size, season and location, as well as the differentiation among stages for the survival dependencies of these factors. The young haploid females survived more than the young of the other stages under Allee effects during the environmentally stressful season at the more exposed locations, and under self-thinning during the active growth season. Furthermore, fertile haploid females had a higher survival than fertile haploid males or fertile diploids.ConclusionsHere, we show a survival advantage of haploids over diploids. The haploid females probably optimize their resource management targeting structural and physiological adaptations that significantly enhance survival under harsher conditions. In a companion paper we demonstrate a fertility advantage of diploids over haploids. Together, the survival and fertility differentiation support the evolution and prevalence of biphasic life-cycles.
Highlights
IntroductionAlmost all algae (green, brown and red) have complex biphasic life-cycles, known as haploid-diploid lifecycles, that alternate between free-living diploid (tetrasporophytes) and haploid (gametophytes) phases
Almost all algae have complex biphasic life-cycles, known as haploid-diploid lifecycles, that alternate between free-living diploid and haploid phases
Ii) very low densities were detrimental to survival; a feature known as Allee effects
Summary
Almost all algae (green, brown and red) have complex biphasic life-cycles, known as haploid-diploid lifecycles, that alternate between free-living diploid (tetrasporophytes) and haploid (gametophytes) phases. Different cytological processes of spore production [11, 12] and niche partitioning through conditional differentiation of their ecophysiological responses to environmental stimuli [5, 6, 8, 13,14,15,16,17,18] have been proposed to lead to unbalanced ploidy rates and abundances in haploid-diploid populations. In most of these studies, the performed analyses were fragmented, focusing only on specific aspects (such as spore viability, resistance to herbivory or photosynthetic performance), and several were based upon laboratory experiments rather than measures under natural conditions They were unable to demonstrate the occurrence in the field of the conditional differentiation among the isomorphic adults that can only be unveiled by an integrative analysis. Stage dependent survival rates are the most efficient way to explain conditional differentiation
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