A century-long genetic record reveals that protist effective population sizes are comparable to those of macroscopic species

Abstract

Effective population size (N e ) determines the rate of genetic drift and the relative influence of selection over random genetic changes. While free-living protist populations characteristically consist of huge numbers of cells (N), the absence of any estimates of contemporary N e raises the question whether protist effective population sizes are comparably large. Using microsatellite genotype data of strains derived from revived cysts of the marine dinoflagellate Pentapharsodinium dalei from sections of a sediment record that spanned some 100 years, we present the first estimates of contemporary N e for a local population in a free-living protist. The estimates of N e are relatively small, of the order of a few 100 individuals, and thus are similar in magnitude to values of N e reported for multicellular animals: the implications are that N e of P. dalei is of many orders of magnitude lower than the number of cells present (N e /N 10 212 ) and that stochastic genetic processes may be more prevalent in protist populations than previously anticipated.

Exploring the impact of multidecadal environmental changes on the population genetic structure of a marine primary producer

“…The results were analyzed using GeneMapper 4.1 software (Applied Biosystems) where null alleles were given the code 1. The main bulk of the raw data was used for estimating effective population sizes of P. dalei (Watts et al., ), and genotypes were archived with Dryad: doi:. The dataset has been reanalyzed and some null alleles determined as belonging to one allele.…”

“…We revived P. dalei resting cysts from dated sediment layers spanning nearly a century, established cultures of vegetative cells (Lundholm et al., ), amplified species‐specific microsatellite markers (Lundholm, Nielsen, Ribeiro, & Ellegaard, ), and explored the data for estimating protist effective populations sizes (Watts et al., ). The sediment cores were retrieved from Koljö Fjord, a sill fjord that has laminated, oxygen‐depleted sediment well suited for the preservation of resting stages.…”

“…How this diversity is maintained is one of the key questions in plankton ecology. For most species, the frequency of sexual reproduction is unknown and recent studies have shown that the effective population sizes may be relatively small compared to the enormous total population size (Watts, Lundholm, Ribeiro, & Ellegaard, 2013). Genetic differentiation related to spatial patterns has been explored in marine protists at both small (Darling, Kucera, & Wade, 2007;Godhe & Härnström, 2010;Lowe et al, 2010;Rynearson & Armbrust, 2004) and larger scales (Casteleyn et al, 2010;Iglesias-Rodriguez, Schofield, Batley, Medlin, & Hayes, 2006).…”

Evolution of mating types in finite populations: The precarious advantage of being rare

“…Comparing our results to empirical observations is further hindered by difficulties in accurately estimating effective population sizes, particularly in microbes. Estimates of the effective population sizes of a single species can vary by several orders of magnitude (Katz, Snoeyenbos‐West, & Doerder, ) but some unicellular protists are thought to have very small effective population sizes: temporal estimates from a single geographical location of the marine dinoflagellate Pentapharsodinium dalei revealed an effective population size in the order of 100 (Watts, Lundholm, Ribeiro, & Ellegaard, ).…”

“…temporal estimates from a single geographical location of the marine dinoflagellate Pentapharsodinium dalei revealed an effective population size in the order of 100 (Watts, Lundholm, Ribeiro, & Ellegaard, 2013).…”