Genotyping an <i>Emiliania huxleyi</i> (prymnesiophyceae) bloom event in the North Sea reveals evidence of asexual reproduction

Abstract

Abstract. Due to the unprecedented rate at which our climate is changing, the ultimate consequence for many species is likely to be either extinction or migration to an alternate habitat. Certain species might, however, evolve at a rate that could make them resilient to the effects of a rapidly changing environment. This scenario is most likely to apply to species that have large population sizes and rapid generation times, such that the genetic variation required for adaptive evolution can be readily supplied. Emiliania huxleyi (Lohm.) Hay and Mohler (Prymnesiophyceae) is likely to be such a species, as it is the most conspicuous extant calcareous phytoplankton species in our oceans with growth rates of 1 day−1. Here we report on a validated set of microsatellites, in conjunction with the coccolithophore morphology motif genetic marker, to genotype 93 clonal isolates collected from across the world. Of these, 52 came from a single bloom event in the North Sea collected on the D366 United Kingdom Ocean Acidification cruise in June–July 2011. There were 26 multilocus genotypes (MLGs) encountered only once in the North Sea bloom and 8 MLGs encountered twice or up to six times. Each of these repeated MLGs exhibited Psex values of less than 0.05, indicating each repeated MLG was the product of asexual reproduction and not separate meiotic events. In addition, we show that the two most polymorphic microsatellite loci, EHMS37 and P01E05, are reporting on regions likely undergoing rapid genetic drift during asexual reproduction. Despite the small sample size, there were many more repeated genotypes than previously reported for other bloom-forming phytoplankton species, including a previously genotyped E. huxleyi bloom event. This study challenges the current assumption that sexual reproduction predominates during bloom events. Whilst genetic diversity is high amongst extant populations of E. huxleyi, the root cause for this diversity and ultimate fate of these populations still requires further examination. Nonetheless, we show that certain CMM genotypes are found everywhere, while others appear to have a regional bias.

Isolation by Time During an Arctic Phytoplankton Spring Bloom

“…clonal) genotypes was reported by Krueger‐Hadfield et al. () who sampled eight genotypes of E. huxleyi from two up to six times during a bloom event in the North Sea. All these studies raise questions about the diversity of mechanisms maintaining genetic diversity, such as the mode of reproduction, and the factors shaping the genetic structure of the blooms.…”

“…clonal) reproduction (of the presumably fittest genotypes), as evident in a bloom of the prymnesiophyte Emiliania huxleyi (Krueger‐Hadfield et al. ), but the importance of sexual reproduction in relation to phytoplankton blooms is almost unknown.…”

Clonal expansion behind a marine diatom bloom

“…High genotypic diversity has generally been detected for aquatic microalgae (Rengefors et al, 2017), including diatoms sampled along a bloom such as Ditylum brightwellii (Rynearson and Armbrust, 2005), Skeletonema marinoi (Godhe et al, 2016) and P. multistriata (Tesson et al, 2014 and this study). Nonetheless, low diversity was observed in some cases: the dominance of asexual reproduction and clonal evolution were considered at the base of the relatively low genotypic richness (0.667) reported for the coccolithophore Emiliania huxleyi during a bloom in the North Sea (Krueger-Hadfield et al, 2014). Extremely low genotypic richness (down to~0) was also reported for a benthic freshwater diatom collected in small pond (Vanormelingen et al, 2015), although the hypothesis of clonal expansion could not be investigated due to the limited number of samples.…”

Swift thermal reaction norm evolution in a key marine phytoplankton species

“…In any case, the standing genetic variation in natural phytoplankton populations is much higher: ample standing genetic variation in coccolithophores has often been observed in genetic marker studies (Iglesias-Rodriguez et al 2006;Krueger-Hadfield et al 2014) or physiological assessments (Brand 1982;Wood and Leatham 1992;Kremp et al 2012;Zhang et al 2014), which provide abundant possibilities for selection to operate.…”