GILBERT, JJ: Two Types of Phenotypic Plasticity Control a Rotifer Life Cycle

In many monogonont rotifers, parthenogenetically-produced diploid embryos are phenotypically plastic in that they may become either amictic or mictic females. The former produce diploid eggs that develop into females, while the latter produce haploid eggs that develop into males or, if fertilized, into resting eggs–diapausing female embryos. In a Florida strain of Brachionus calyciflorus, diploid eggs develop into mictic females only when their mothers are crowded. The crowding stimulus appears to be a strain-specific chemical that is released into the environment and that affects some oocytes within the mother either directly or indirectly by altering the maternal physiology. The partial response to crowding, where only some eggs develop into mictic females, appears to be a bet-hedging strategy assuring production of resting eggs under favorable conditions while still permiting population growth via female parthenogenesis. In this strain of B. calyciflorus, a transgenerational phenotypic plasticity determines the propensity of amictic females to produce mictic daughters in response to crowding. The proportion of diploid eggs becoming mictic females is very low in females hatching from resting eggs and gradually increases to about 0.5 after about 12 generations of female parthenogenesis. The mechanism for this fertilization-induced, parental effect may be a chemical from the yolk gland of fertilized mictic females that inhibits development into mictic females and that is transmitted from one parthenogenetic generation to the next in increasingly low concentrations. The phenomenon ensures that a clone developing from a resting egg will continue to increase its population size before maximizing its commitment to sexual reproduction, thus increasing the number of resting eggs to which it contributes its genes.