Abstract
Whole genome duplication is now accepted as an important evolutionary force, but the genetic factors and the life history implications affecting the existence and abundance of polyploid lineages within species are still poorly known. Polyploidy has been mainly studied in plant model species in which the sporophyte is the dominant phase in their life history. In this study, we address such questions in a novel system (Porphyra, red algae) where the gametophyte is the dominant phase in the life history. Three Porphyra species (P. dioica, P. umbilicalis, and P. linearis) were used in comparisons of ploidy levels, genome sizes and genetic differentiation using flow cytometry and 11 microsatellite markers among putative polyploid lineages. Multiple ploidy levels and genome sizes were found in Porphyra species, representing different cell lines and comprising several cytotype combinations among the same and different individuals. In P. linearis, genetic differentiation was found among three polyploid lineages: triploid, tetraploid and mixoploids, representing different evolutionary units. We conclude that the gametophytic phase (n) in Porphyra species is not haploid, contradicting previous theories. New hypotheses for the life histories of Porphyra species are discussed.
Highlights
Polyploidy, the increase in genome size by the acquisition of more than one set of chromosomes has been a key factor in eukaryote evolution
The minimum ploidy level was set in the nuclei population with the lowest genome size, which was found in the gametes (DNA corresponding to the non-replicated haploid chromosome complement)
121 multi-locus genotypes were found (Appendix 2). These where divided in 19 groups according to the species and ploidy level found in the flow cytometric analyses
Summary
Polyploidy, the increase in genome size by the acquisition of more than one set of chromosomes has been a key factor in eukaryote evolution. Most of the systems studied are higher plants (e.g. Brassica, Cotton, Maize, Spartina, Wheat, Tragopogon) representing life cycles in which the sporophyte is the dominant phase in a complex cycle. This cycle alternates between sporophytes (spore-producing organisms) and gametophytes (gamete-producing organisms)[9]. Drew proved that Conchocelis rosea Batters, originally described as a separate species, was the germinated carpospores (sporophyte, phase 2n) from the Porphyra blades (gametophyte, phase n) It has been assumed since than two ploidy levels are involved in the Porphyra life cycles and several chromosome studies have corroborated the ploidy level of both phases, with the gametophytic blade being haploid and the sporophytic carpospores/conchocelis being diploid (e.g. refs 22–25). Drew’s study enabled mass cultivation and the development of the aquaculture industry of Porphyra (Nori, laverbread), the single most valuable marine product in the orient with a current retail value over $1.3 billion per year[26]
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