Narrow roads to Fern Land: revisiting the paradox of sexual reproduction

Joachim L. Dagg 

Following some major thought on the evolutionary maintenance of sex leads to a new hypothesis about the role of life cycles. Organisms with a heterogonic life cycle, like strawberries, propagate contrary to what would be adaptive under red-queen selection from micro-parasites. Their recombinant offspring disperses, but their clonal offspring stays close to the parent. In diplohaplontic organisms, like ferns, fertilisation and meiosis occur in different generations. Recombination is spread over the whole life cycle. Their zygotes grow on the spot of their maternal gametophyte and are recombinant through syngamy. The resulting sporophytes produce dispersing spores, which are recombinant through meiosis. This should better adapt ferns to red-queen selection than strawberry-kin species (Potentilleae). 

Phylogenetic generalised least squares analyses show that the number of parasitic fungi recorded per species rises significantly with the number of citations per species both for Potentilleae and ferns. The slopes also differed significantly from each other and that for strawberry-kin was steeper. Organisms with a strawberry-like life cycle should do better with staying recombinant offspring and dispersing clonal offspring. This would amount to sexually producing runners, tubers, polyps and may well-nigh be impossible. Theoretically, life cycles with recombinant offspring that stay and clonal offspring that disperse should be best adapted against red-queen selection from micro-parasites. The rarity of this ‘red-queen life cycle’ among multicellular species remains perplexing.