Why reproduction often takes two? Why have sex with another when selfing is easier and less costly? ... Adaptation Favours Outcrossing

It is an evolution mystery - self-fertilization (selfing) is the efficient way to breed; when it comes to reproduction, the nature still prefers outcrossing.

Recent study in worm suggests that outcrossing shows significantly more rapid adaption to selection.

Nature. 2009 Nov 19;462(7271):350-2. Epub 2009 Oct 21.

Mutation load and rapid adaptation favour outcrossing over self-fertilization.

Morran LT, Parmenter MD, Phillips PC.

The tendency of organisms to reproduce by cross-fertilization despite numerous disadvantages relative to self-fertilization is one of the oldest puzzles in evolutionary biology. For many species, the primary obstacle to the evolution of outcrossing is the cost of production of males, individuals that do not directly contribute offspring and thus diminish the long-term reproductive output of a lineage. Self-fertilizing ('selfing') organisms do not incur the cost of males and therefore should possess at least a twofold numerical advantage over most outcrossing organisms. Two competing explanations for the widespread prevalence of outcrossing in nature despite this inherent disadvantage are the avoidance of inbreeding depression generated by selfing and the ability of outcrossing populations to adapt more rapidly to environmental change. Here we show that outcrossing is favoured in populations of Caenorhabditis elegans subject to experimental evolution both under conditions of increased mutation rate and during adaptation to a novel environment. In general, fitness increased with increasing rates of outcrossing. Thus, each of the standard explanations for the maintenance of outcrossing are correct, and it is likely that outcrossing is the predominant mode of reproduction in most species because it is favoured under ecological conditions that are ubiquitous in natural environments.

PMID: 19847164

The way the authors used to measure the fitness is：
“We used a competitive fitness assay to measure the change in fitness for each experimental population relative to its ancestor before selection. The competitive fitness assays were conducted within the context of the selective environment and the assay was conducted simultaneously on the experimental population and the previously frozen ancestral population. Fitness was determined by mixing each population (experimental and ancestral) with a GFP-marked tester strain at a 50:50 ratio. After passaging the worms in the relevant selective environment, the GFP ratio of the offspring was calculated and used to estimate fitness.”

F1000 review

John Logsdon
University of Iowa, United States of America
Genomics & Genetics

Why have sex with another when sex with yourself is easier and less costly? For hermaphroditic organisms capable of self-fertilization, this is an important question. Clear experimental answers are provided by this paper -- and they are exactly in line with previous theory: outcrossing accelerates adaptation and reduces mutational impacts.

The nematode Caenorhabditis elegans exists as a hermaphrodite and as a male, having wild-type outcrossing rates (male x hermaphrodite) of usually less than 5%. In this study, the authors introduced known mutations to create both obligate outcrossers and obligate selfers in otherwise identical genetic backgrounds. Selfers should purge mutations effectively, but outcrossers are expected to fare better when new mutations are frequent and their effects are weak. In a 50-generation evolution experiment in which mutation rate was increased by 4-fold and a novel environment was imposed to allow strong selection against deleterious mutations, the obligate selfers fixed many more (deleterious) mutations. The outcrossers stayed fit while the selfers showed a marked fitness decline. The outcrossers also showed a significantly more rapid response -- adaptation -- to selection. As a further test of the adaptive response, the authors designed an experiment that imposed very strong selection; worms were exposed to the highly virulent bacterium, Serratia marcescens. Under this adapt-or-die scenario, only the outcrossers adapted to the pathogen. This work provides insight into why obligate self-fertilization is rarely observed in nature and why outcrossing is maintained, despite its costs. More studies will be needed to validate the conclusions in natural conditions, and this study paves the way. Finally, nice parallels can be drawn with sexual and asexual species, the latter of which may be doomed to extinction. In the end, obligate selfing may cost the same as not having sex at all.