Variation in the life cycle of monogonont rotifers: Commitment to sex and emergence from diapause

Abstract

A review of research on life‐cycle events in field and laboratory populations of monogonont rotifers shows that there is great variation at multiple levels: (1) degree of sexual dimorphism; (2) occurrence and timing of sex; (3) propensity for sex during sexual periods; (4) factors controlling initiation of sex; and (5) timing and extent of emergence from diapause. There is no regular pattern where: (1) fertilised resting eggs hatch to start the growing season; (2) populations develop via female parthenogenesis during favourable conditions; and then (3) bisexual reproduction with resting‐egg production occurs during later, unfavourable conditions. Sexual reproduction in natural populations can occur throughout much of the growing season, be restricted to some period(s) during the growing season, or be completely absent. During sexual reproduction in both natural and laboratory populations, only some fraction of females produces males or resting eggs. This bet‐hedging strategy can prevent a population crash and permits future population growth via female parthenogenesis. Selection against sexual reproduction, and rapid loss of sex, can occur. Laboratory experiments with pond‐dwelling species have identified specific environmental factors that induce sex in different species: (1) increasing population density; (2) dietary tocopherol (vitamin E) and (3) long photoperiods. These factors generally are associated with favourable conditions for population growth and production of energy‐rich resting eggs: (1) large population size; (2) high probability of contacts between males and fertilisable females; and (3) nutritious diets. Endogenous factors can inhibit responses to these environmental inducers, and thus favour female parthenogenesis. The timing of resting‐egg hatching depends on: (1) occurrence of specific environmental conditions; (2) the minimum duration of obligate diapause; and (3) the genotype and physiology of females producing resting eggs. Hatching may occur shortly after oviposition, after a long diapause before or at the start of a new growing season, or throughout the growing season. Hatching can be massive and contribute substantially to population growth and genetic diversity. Areas for future research include: (1) determining the timing and extent of sex and resting‐egg hatching in more natural populations, especially those that are marine, benthic, sessile, and interstitial; and (2) identifying environmental and physiological factors controlling these events.

Lifespan Extension in Long-Lived Vertebrates Rooted in Ecological Adaptation

“…Hypoxia studies in the rotifer Brachionus manjavacas, an aquatic invertebrate suitable for aging studies, demonstrated a mean lifespan extension of 107% when continuously exposed to 1.6% atm. Hypoxic conditions in this organism increased reproductive success by twofold and conferred cytoprotection against stress (Snell et al, 2019). This observation supports previous studies on rotifer communities and their adaptation to high salinity and low oxygen (Esparcia et al, 1989).…”

Density, parasitism, and sexual reproduction are strongly correlated in lake Daphnia populations

“…Our findings are consistent with earlier studies that found density to be an important factor influencing the shift from asexual to sexual reproduction in cyclical parthenogens like Daphnia and rotifers (Berg et al, 2001;Gilbert, 2020;Haltiner et al, 2020;Larsson, 1991;Stelzer & Snell, 2003;Stross & Hill, 1965). One possible explanation for this association is that, in many cyclical parthenogens, sexual reproduction is associated with the production of long-lasting resting…”

“…stages, meaning sexual reproduction may serve as a means of temporal dispersal when faced with strong competition in dense populations Gilbert, 2020). High densities also reduce the relative costs of sexual reproduction; as populations approach carrying capacity, asexual reproduction is less beneficial, reducing the opportunity costs of sexual reproduction (Burt, 2000;.…”

Seasonal variation in sexual readiness in a facultatively sexual freshwater cnidarian with diapausing eggs

“…The switch from asexual to sexual reproduction in facultatively sexual animals can be highly variable within and between species (Tessier and C aceres 2004, Navarro et al 2013, Franch-Gras et al 2017, Ryan and Miller 2019. For instance, in freshwater invertebrates such as water fleas, monogonont rotifers or freshwater hydra clonal lineages frequently differ in sexual propensity such that some genotypes show no sexual investment at all, while in others all individuals within a clonal lineage invest in sex in response to specific environmental cues (Tessier and C aceres 2004, T€ ok€ olyi et al 2017b, Gilbert 2020. Central to research on facultatively sexual organisms is to identify factors that drive this variation.…”