Obligate asex in a rotifer and the role of sexual signals

Abstract

Transitions to asexuality have occurred in many animals and plants, yet the biological mechanisms causing such transitions have often remained unclear. Cyclical parthenogens, such as cladocerans, rotifers or aphids often give rise to obligate asexual lineages. In many rotifers, chemical signals that accumulate during population crowding trigger the induction of sexual stages. In this study, I tested two hypotheses on the origin of obligate parthenogenesis in the rotifer Brachionus calyciflorus: (i) that obligate parthenogens have lost the responsiveness to the sexual signal; and (ii) that obligate parthenogens have lost the ability to produce the sexual signal. Pairwise cross‐induction assays among three obligate parthenogenetic strains and two cyclically parthenogenetic (sexual) strains were used to test these hypotheses. I found that obligate parthenogens can induce sexual reproduction in sexual strains, but not vice versa. This demonstrates that obligate parthenogens do still produce the sexual signal, but have lost responsiveness to that signal.

A first assessment of genome size diversity in Monogonont rotifers

“…The Florida strain of B. calyciflorus was obtained from J.J. Gilbert (Dartmouth College, NH, USA) and has been characterized in Gilbert & Walsh (2005). The Egelsee strain of B. calyciflorus was isolated from Lake Egelsee (Austria) and was also used in previous studies (e.g., Stelzer, 2008). The Russian strain of B. plicatilis has long been used in various laboratory studies (e.g., Snell & Stelzer, 2005) and was originally isolated from the Azov Sea (Russia).…”

Loss of Sexual Reproduction and Dwarfing in a Small Metazoan

“…Monogononts normally reproduce by cyclical parthenogenesis, an alternation between ameiotic parthenogenesis and sporadic sexual episodes [13]. But there are also several documented cases on Brachionus strains that have permanently lost the ability to reproduce sexually [14], [15], [16], [17], [18]. In cyclical parthenogenetic rotifers, sex is initiated with the production of sexual females, whose oocytes undergo meiosis and develop into haploid males (if not fertilized), or diploid diapausing eggs (if fertilized).…”

“…Obligate parthenogens are not able to produce sexual females, thus they also lack males and diapausing eggs. It has been demonstrated that this inability is caused by a loss of responsiveness to the chemical signal that induces sex [18]. As a consequence, populations of obligate parthenogens can grow to extremely high population densities, without ever inducing sex, whereas cyclical parthenogens readily induce sexual reproduction as soon as population densities exceed one female per ml [23].…”

Neural control of gene recruitment in metazoans

“…The water flea, Daphnia magna (Strauss, 1820), is a cyclic parthenogenetic crustacean that normally gives birth to female offspring only but when environmental conditions deteriorate (crowding, depletion of food resources) or presage deterioration (shortening of the photoperiod) (Stelzer, 2008), it adaptively produces a sexually reproducing generation (male þ female individuals). At the molecular level the transition is triggered by neurally controlled secretion of methyl farnesoate, Daphnia's juvenoid hormone.…”