Mating dynamics in a nematode with three sexes and its evolutionary implications

Nematodes have diverse reproductive strategies, which make them ideal subjects for comparative studies to address how mating systems evolve. Here we present the sex ratios and mating dynamics of the free-living nematode Rhabditis sp. SB347, in which males, females and hermaphrodites co-exist. The three sexes are produced by both selfing and outcrossing, and females tend to appear early in a mother’s progeny. Males prefer mating with females over hermaphrodites, which our results suggest is related to the female-specific production of the sex pheromones ascr#1 and ascr#9. We discuss the parallels between this system and that of parasitic nematodes that exhibit alternation between uniparental and biparental reproduction

Heritable determinants of male fertilization success in the nematode Caenorhabditis elegans

<p>Abstract</p> <p>Background</p> <p>Sperm competition is a driving force in the evolution of male sperm characteristics in many species. In the nematode <it>Caenorhabditis elegans</it>, larger male sperm evolve under experimentally increased sperm competition and larger male sperm outcompete smaller hermaphrodite sperm for fertilization within the hermaphrodite reproductive tract. To further elucidate the relative importance of sperm-related traits that contribute to differential reproductive success among males, we quantified within- and among-strain variation in sperm traits (size, rate of production, number transferred, competitive ability) for seven male genetic backgrounds known previously to differ with respect to some sperm traits. We also quantified male mating ability in assays for rates of courtship and successful copulation, and then assessed the roles of these pre- and post-mating traits in first- and second-male fertilization success.</p> <p>Results</p> <p>We document significant variation in courtship ability, mating ability, sperm size and sperm production rate. Sperm size and production rate were strong indicators of early fertilization success for males that mated second, but male genetic backgrounds conferring faster sperm production make smaller sperm, despite virgin males of all genetic backgrounds transferring indistinguishable numbers of sperm to mating partners.</p> <p>Conclusions</p> <p>We have demonstrated that sperm size and the rate of sperm production represent dominant factors in determining male fertilization success and that <it>C. elegans </it>harbors substantial heritable variation for traits contributing to male reproductive success. <it>C. elegans </it>provides a powerful, tractable system for studying sexual selection and for dissecting the genetic basis and evolution of reproduction-related traits.</p

Some similarity states of stably stratified homogeneous turbulence

The decay of statistically homogeneous velocity and density fluctuations in a stably stratified fluid is considered. Over decay times long compared with the turbulence time scale but short compared with the period of internal gravity waves, three distinct high Reynolds number similarity states may develop. These similarity states are a consequence of the invariance of the low wavenumber coefficients of the three-dimensional kinetic or potential energy spectrum, and their preferential development depends on the relative magnitudes of the initial kinetic and potential energy per unit mass of the fluid. When the turbulence has decayed over a time comparable with the period of the gravity waves, the three similarity states mentioned above are disrupted. Evidence will be presented of a new similarity state which then develops asymptotically. In this similarity state, the time decay exponent of the total energy per unit mass of the turbulence is reduced by a factor of two from its value for decaying isotropic turbulence, and the associated vertical integral scale approaches a constant independent of time

On the decay of inhomogeneous turbulence

The decay of high-Reynolds-number inhomogeneous turbulence in an unbounded domain is considered. The turbulence may be initially localized in one to three spatial directions and the fluid is assumed to be at rest at infinity in those directions, Previous arguments used to determine the decay laws of homogeneous turbulence are extended to the decay of inhomogeneous turbulence by integrating the turbulence statistics over the inhomogeneous directions. Dimensional arguments based on the invariance or near-invariance of low-wavenumber spectral coefficients associated with the integrated mean-square velocity are used to determine asymptotic decay laws for inhomogeneous turbulence. These decay laws depend on the number of inhomogeneous directions of the flow field and reduce to the well-known decay laws of homogeneous turbulence when this number is zero. Different decay laws are determined depending on the spectral behaviour at low wavenumbers. Asymptotic similarity states of the spectrum during the decay and of the distribution of the mean-square velocity along the inhomogeneous directions are also determined. An analytical result for the decay of the mean-square velocity at the centre of the initial disturbance is found, and the decay proceeds more rapidly with increasing number of inhomogeneous directions due to the transport of energy along those directions. Large-eddy simulations of decaying turbulence homogeneous in a plane and localized in a single direction are performed to test the theoretical scaling laws. The numerically determined asymptotic decay laws of the integrated mean-square velocity agree well with the theoretical predictions. A self-similar decay of the spectra and mean-square velocity distributions is also observed. The simulation results suggest that when the low-wavenumber spectral coefficient is an exact invariant, a unique similarity state depending only on the initial value of this invariant and independent of all other aspects of the initial conditions is attained asymptotically

The viscous-convective subrange in nonstationary turbulence

The similarity form of the scalar-variance spectrum at high Schmidt numbers is investigated for nonstationary turbulence. Theoretical arguments show that Batchelor scaling may apply only at high Reynolds numbers. At low Reynolds numbers, Batchlor scaling is not possible unless the turbulence is stationary or the enstrophy decays asymptotically as t(-2). When this latter condition is satisfied, it is shown from an analysis using both the Batchelor and Kraichnan models for the scalar-variance transfer spectrum that the k(-1) power law in the viscous-convective subrange is modified. Results of direct numerical simulations of high Schmidt number passive scalar transport in stationary and decaying two-dimensional turbulence are compared to the theoretical analysis. For stationary turbulence, Batchelor scaling is shown to collapse the spectra at different Schmidt numbers and a k(-1) viscous-convective subrange is observed. The Kraichnan model is shown to accurately predict the simulation spectrum. For nonstationary turbulence decaying at constant Reynolds number for which the enstrophy decays as t(-2), scalar fields for different Schmidt numbers are simulated in situations with and without a uniform mean scalar gradient. The Kraichnan model is again shown to predict the spectra in these cases with different anomalous exponents in the viscous-convective subrange. (C) 1998 American Institute of Physics

On the decay of two-dimensional homogeneous turbulence

Direct numerical simulations of decaying two-dimensional turbulence in a fluid of large extent are performed primarily to ascertain the asymptotic decay laws of the energy and enstrophy. It is determined that a critical Reynolds number R(c) exists such that for initial Reynolds numbers with R(0)<R(c) final period of decay solutions result, whereas for R(0)>R(c) the flow field evolves with increasing Reynolds number. Exactly at R(0)=R(c), the turbulence evolves with constant Reynolds number and the energy decays as t(-1) and the enstrophy as t(-2). A t(-2) decay law for the enstrophy was originally predicted by Batchelor for large Reynolds numbers [Phys. Fluids Suppl. II, 12, 233 (1969)]. Numerical simulations are then performed for a wide range of initial Reynolds numbers with R(0)>R(c) to study whether a universal power-law decay for the energy and enstrophy exist as t-->infinity. Different scaling laws are observed for R(0) moderately larger than R(c). When R(0) be comes sufficiently large so that the energy remains essentially constant, the enstrophy decays at large times as approximately t(-0.8). (C) 1997 American Institute of Physics

Mutation selection balance, dominance and the maintenance of sex

A leading hypothesis for the evolutionary function of sex postulates that sex is an adaptation that purges deleterious mutations fi om the genome, thereby increasing the equilibrium mean fitness of a sexual population relative to its asexual competitor. This hypothesis requires two necessary conditions: first, the mutation rate pet genome must be of order one, and, second, multiple mutations within a genome must act with positive epistasis, that is, two or more mutations of different genes must he more harmful together than if they acted independently. Here, by reconsidering the theory of mutation-selection balance at a single diploid gene locus, we demonstrate a significant advantage of sex due to nearly recessive mutations provided the mutation rate per genome is of order one. The assumption of positive epistasis is unnecessary, and multiple mutations may he assumed to act independently

A Fourier-Hermite pseudospectral method for penetrative convection

A Fourier-Hermite pseudospectral method is developed to study numerically the three-dimensional penetrative convection problem under the Boussinesq approximation. An S-shaped temperature profile in the absence of motion is prescribed in the vertical direction. All variables are expanded in terms of Fourier-Hermite basis functions. The Hermite functions are scaled to adjust the length of the computational domain in the vertical. A semi-implicit scheme is used for time marching with the third-order Adam-Bashforth and Crank-Nicholson scheme for the nonlinear and linear terms, respectively. An implementation of the numerical method on a parallel computer is also described. Numerical simulation results of resolution 64(3) are presented for low-to-moderate Rayleigh numbers with a Prandtl number of unity. The highly stable outer regions are seen to act as effective lids and all penetrative flow are contained within the computational box. Variances, heat fluxes, and their budgets are reported for several Rayleigh numbers to demonstrate the efficacy of the numerical method. (C) 1998 Academic Press