Differences in response between haploid and diploid isomorphic phases of Gracilaria verrucosa (Rhodophyta: Gigartinales) exposed to artificial environmental conditions

This study tests the responses of juvenile gametophytes and tetrasporophytes (holdfast stage) of the isomorphic alga Gracilaria verrucosa under different environmental conditions.Estimations of survival and growth of holdfasts of haploid and diploid juvenile individuals were performed in natural sea-water and artificial culture medium, and under stringent conditions using lead as a toxin and ultra violet radiation as a mutagen. Results indicate that (i) holdfasts of haploid juveniles grow better than diploids in non-optimal medium conditions; (ii) holdfasts of diploid juveniles have a better tolerance to lead than haploids; and (iii) slight advantage of holdfasts of diploid juveniles grow better than haploids under U.V. radiation.     

Variation of reproductive success in a haplo-diploid Red Alga,Gracilaria Verrucosa: effects of parental identities and crossing distance

The factors influencing reproductive success in a haplo-diploid marine alga, Gracilaria verrucosa, have been determined through single-male crosses in the laboratory. Crossing success was assessed by measuring its early components, fertility, and abortion rate. The effects of the male or female parent identity, of the male × female interaction, and of the geographical distance between mates were tested in crosses within or between populations at different geographical scales. The identity of the female parent has a predominant effect on crossing success. Interparent distance has no effect in within-population crosses; this strongly suggests an absence of inbreeding depression, expected in a species where the haploid phase is individualized and isomorphic to the diploid one. Crossing success tends to increase with distance between mates in between-population crosses. This heterosis effect seems to indicate that the marine environment could allow genetic divergence between populations, even at short distances (about 100 m).     

Evolution of alternation of haploid and diploid phases in life cycles

Eukaryotic sex leads to an alternation of haploid and diploid nuclear phases. Because all multicellular animals are diploid, diploidy is often considered a 'biological success' and many arguments have been advanced to explain the evolution of a prolonged diploid phase. Nevertheless, among eukaryotes three basic situations are encountered, where the vegetative individuals are diploid or haploid or both. These three basic life cycles are widely distributed among kingdoms and in some taxa the occurrence of different life cycles within the same species has been reported. This article briefly summarizes the different hypotheses on the evolution of reproductive life cycles and underlines how possibilities of variation for this trait may open new perspectives for research.     

Evolution of the alternation of haploid and diploid phases in life cycles. II. Maintenance of the haplo-diplontic cycle

The relative duration of the haploid and the diploid phases during the reproductive cycle varies greatly between organisms. This paper addresses the question of the evolution of haploid, diploid, and haplo-diplontic life cycles. When the life span of haploid and diploid individuals is constant whatever their cycle, we show that the haplo-diplontic cycle has an advantage, which depends on the sex-ratio in anisogamous species and on the probability of fertilization in isogamous species. This is because meiosis and fertilization occur half as often in the haplo-diplontic cycle as in haploid or diploid cycles, for the same number of generations of individuals. This argument is demonstrated using a model which considers a genetic determination of the cycle, and fixed haploid and diploid fitnesses. The relevance of measures of fitness of haploid and diploid individuals in predicting the evolution of life cycles is discussed. Measures obtained in algae are compared with theoretical predictions.