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PubMed2003

Public health policy and infertility.

In assisted reproductive technology, three main areas are relevant from a health policy perspective: first, the technical excellence of the services rendered; second, a licensing and monitoring system to control all practitioners and the premises in which they work; and third, legislation to avoid abuse, prevent possible damage to the mother and the child and outlaw techniques unacceptable in a given cultural setting, including mechanisms to ensure the adherence by all to ethical and deontological principles. No one refuses the need to ensure that services offering assisted reproductive technologies adhere to technical excellence and everyone agrees that technical guidelines should exist and should be thoroughly enforced. When it comes to legislating about assisted reproductive techniques and, more specifically, to ruling in favour of limiting the use of existing technology to certain groups and not to others, or to ban altogether individual methods, views are sometimes totally divergent. In countries where legislation is fairly liberal, usually both physicians and the public view legislation with favour; conversely, where a number of restrictions exist, many complain that legislation impairs their ability to offer proper services. The solution seems to be a simple, well-defined, legislative act setting clear principles and providing for the creation of an Authority capable of resolving the myriad of individual problems that invariably arise from applying assisted reproductive techniques. Homo sum et nihil humanum a me alienum puto I am a human being and therefore cannot consider alien to me anything that helps another human being.

Female↗
PubMed1993

The advance of Muller's ratchet in a haploid asexual population: approximate solutions based on diffusion theory.

Asexual populations experiencing random genetic drift can accumulate an increasing number of deleterious mutations, a process called Muller's ratchet. We present here diffusion approximations for the rate at which Muller's ratchet advances in asexual haploid populations. The most important parameter of this process is n0 = N e-U/s, where N is population size, U the genomic mutation rate and s the selection coefficient. In a very large population, n0 is the equilibrium size of the mutation-free class. We examined the case n0 > 1 and developed one approximation for intermediate values of N and s and one for large values of N and s. For intermediate values, the expected time at which the ratchet advances increases linearly with n0. For large values, the time increases in a more or less exponential fashion with n0. In addition to n0, s is also an important determinant of the speed of the ratchet. If N and s are intermediate and n0 is fixed, we find that increasing s accelerates the ratchet. In contrast, for a given n0, but large N and s, increasing s slows the ratchet. Except when s is small, results based on our approximations fit well those from computer simulations.

Computer Simulation↗
PubMed2001

Transposable element number in mixed mating populations.

Theoretical population genetic studies of transposable elements focus almost exclusively on random mating species, whereas many plants reproduce through partial or substantial self-fertilization. Here I develop computer simulation and analytic approximations of simplified element dynamics (transposition balanced by selective elimination) in partially self-fertilizing populations, using Ty1-copia elements for biological inspiration. Under the most plausible models and parameter values, element numbers decrease with self-fertilization when element insertions are deleterious, but may increase when ectopic exchange regulates element number. Conclusions for models of ectopic exchange depend in part on parameters for which little firm empirical evidence is available. Small changes in selfing rate can lead to abrupt changes in element number when homozygous and heterozygous elements have markedly different fitness effects. Equilibrium element numbers can be sensitive to population size, especially at high selfing rates. Elements are frequently lost in small highly selfing populations under the deleterious insertion model. In contrast, small highly selfing populations can accumulate very large numbers of elements under ectopic exchange. Empirical data on element number and localization in plants with different mating systems suggests that deleterious insertion, rather than ectopic exchange, may regulate element number. Limitations to available empirical data, especially the lack of comparison between closely related species differing in mating system, mean that this conclusion is tentative.

Breeding↗
PubMed2004

How the sperm lost its tail: the evolution of aflagellate sperm.

The typical sperm is comprised of a head, midpiece and flagellum. Around this theme there is an enormous diversity of form--giant sperm, multi-flagellate sperm and also sperm that lack flagella entirely. Explaining this diversity in sperm morphology is a challenging question that evolutionary biologists have only recently engaged in. Nonetheless, one of the selective forces identified as being an important factor in the evolution of sperm form is sperm competition, which occurs when the sperm of two or more males compete to fertilize a female's ova. In species with a truly monandrous mating system, the absence of sperm competition means that the selection pressure on males to produce motile sperm may be relaxed. Potentially aflagellate sperm are less costly to produce, both in terms of energy and time. Thus, selection may therefore favour the loss of the sperm flagellum and any other motile mechanisms in monandrous taxa. A review of the literature revealed that 36 taxonomic groups, from red algae to fish, were found independently to have evolved aflagellate sperm. I review what is known about the mating systems of each of these taxa and their nearest sister taxa. A sister-group analysis using this information provided weak evidence suggesting that the evolution of aflagellate sperm could be linked to the removal of selective pressures generated by sperm competition.

Animals↗
PubMed1998

A chromosomal analysis of the phenotypic plasticity of some life history traits in relation to developmental temperature.

The capacity to delay egg deposition in D. melanogaster females in the absence of a sexual partner is genetically determined and opposite types can be artificially selected. In natural populations, the relative frequency of these genotypes varies geographically and seasonally, with temperature as a selective factor. However, the retention duration of these genotypes can be modified by developmental temperature change. To study the genetic control of this response, chromosome substitution between opposite types of line was carried out in order to produce every possible homozygous chromosomal combination of the three major chromosomes (X,2,3). Eggs of these eight constructed lines were developed at two different temperatures (25 degrees C and 14 degrees C). Low temperature development directly affected the number of ovarioles but also modified the subsequent expression of adult characteristics such as retention duration and fecundity. The comparison of the eight lines revealed that, although the 3 chromosomes were involved in the genetic determinism of each trait, only one or two of them were sensitive to temperature change, and these differed according to the trait. For retention duration and fecundity, the effect of chromosome 3 from the long retention strain was particularly affected by low temperature, showing antagonism between the selective effect detected in natural populations and the effect on phenotypic plasticity studied here.

Animals↗