Optimal rates of bisexual reproduction in cyclical parthenogens with density-dependent growth

Abstract

This work explores theoretical patterns of reproduction that maximize the production of resting eggs and the long‐term fitness of genotypes in cyclical parthenogens. Our focus is on density‐dependent reproduction as it influences the consequences of a trade‐off between producing amictic daughters – which reproduce parthenogenetically and subitaneously – and producing mictic daughters – which undergo meiosis and bisexual reproduction. Amictic females increase competitive ability and allow the population to achieve a larger size; mictic females directly contribute to population survival through harsh periods by producing resting eggs. Although morphologically indistinguishable, the two types of females differ greatly in their ecological and reproductive roles. What factors underlie the differential allocation of resources to produce amictic and mictic females? Using a demographic model based on readily accessible parameters we demonstrate the existence of a frequency of mictic females that will maximize the population's long‐term fitness. This frequency, termed the optimal mictic ratio, mo, is 1 − (q/b)1/2, where q is the mortality rate and b is the maximum birth rate. Using computer simulation we compared the fitness of a population with this constant mictic ratio with populations having multiple switches from complete parthenogenetic growth to complete allocation in mixis (mictic ratio either 0 or 1). Two important conclusions for optimal mixis in density‐dependent growth conditions are: (1) intermediate mictic ratios are optimal, and (2) optimal mictic ratios are higher when habitat conditions are better. Physiological cues responding to differences in birth and death rates are common so that it is possible that populations may adjust their relative rates of mictic and amictic female production in response to environmentally induced changes to the optimum mictic ratio. Our analysis demonstrates that different patterns of mixis are expected in different type of habitats. Since the optimal mictic ratio is sensitive to the effects of a variety of environmental challenges, our model makes possible a new means to evaluate life history evolution in cyclical parthenogens.

Diapause as escape strategy to exposure to toxicants: response of Brachionus calyciforus to arsenic

“…In the case of sexual reproduction, optimal timing of initiation is likely dependent on several factors, such as habitat deterioration (e.g. Serra and Carmona 1993;Carmona et al 1993), resource demand (Snell and Boyer 1988;Serra and King 1999) and male-female encounters (Snell and Garman 1986). Sex initiation is density-dependent in the genus Brachionus, triggered by a threshold concentration of infochemical released by the organisms themselves.…”

The expression pattern of dormancy-associated genes in multiple life-history stages in the rotifer Brachionus plicatilis

“…The hatched eggs usually develop into asexually reproducing amictic females (FA), until they encounter a mixis signal directing them to embark on sexual reproduction. The factors inducing the mixis signal in the Brachionus species complex are largely unknown, although population density, salinity, temperature, presence of pheromones and food availability were found to play a role (Lubzens et al, 1985;Lubzens & Zmora, 2003;Serra & King, 1999;Gilbert, 2004aGilbert, , 2007Hagiwara et al, 2005;Snell et al, 2006;Denekamp et al, 2009).…”

Modes, mechanisms and evidence of bet hedging in rotifer diapause traits

“…The timing of sex has been proposed as an instance of the conservative bet-hedging trait in rotifer populations inhabiting unpredictable environments (Carmona et al, 1995;Serra & King, 1999;Schröder & Gilbert, 2004;Serra et al, 2004;Gilbert, 2007a). If the end of the growing season-or the nutritional conditions required for diapausing egg production (Gilbert, 2010)-cannot be predicted by rotifers, then producing diapausing eggs as soon as possible can avoid the risk of a zero-fitness event.…”

“…Type of study: T theoretical, A among-populations empirical study, W within-population empirical study * This species is not E. senta sensu stricto, but a species belonging to the E. senta-species complex as described by 2007; Fussmann et al, 2007). Consequently, intermediate sexual reproduction ratios have been regarded as evidence of a diversified bet-hedging strategy (sensu Seger & Brockmann, 1987) that would allow for the maintenance of both reproductive modes within a clonal lineage (Serra & King, 1999). Intermediate sexual reproduction ratios may evolve in unpredictable habitats, where early sex ensures the production of at least some diapausing eggs.…”

“…Diapausing eggs are the only link between one season and the next; therefore, the number of diapausing eggs produced at the end of a growing season is typically considered the appropriate measure of fitness in rotifer clonal lineages (Serra & King, 1999;Serra et al, 2004;Campillo et al, 2011). Interestingly, many rotifer populations from temperate latitudes live in temporary habitats (Walsh et al, 2014), which are often characterized by year-to-year fluctuations in the length of the planktonic growing season (García-Roger et al, 2014).…”