Pouchkina-Stantcheva et al. 5 have uncovered one possible counter-advantage - distinct functional divergence in the alleles of a gene. They showed recently that the bdelloid rotifer Adineta ricciae carries two alternative versions of a gene that is associated in other organisms with desiccation tolerance (a late embryogenesis abundant gene, with forms Ar-lea-1A and Ar-lea-1B). The two copies, which appear to be allelic, have diverged in sequence by 13.5% of their synonymous sites (that is, sites with 'silent' nucleotide substitutions), 12 of 136 aligned amino-acid positions, and by 132-bp indel (insertion/deletion) within an exon. Southern hybridization and fluorescence in situ hybridization indicated that only two recognizable copies of the Ar-lea gene were present in the genome. Marked sequence divergence among presumed alleles in bdelloids had been found previously in two other genes 6. What distinguishes the gene investigated by Pouchkina-Stantcheva et al. 5 is the functional divergence between the gene copies. One copy codes for a protein that prevents desiccation-sensitive enzymes from aggregating during drying, whereas the other copy encodes a protein that lacks this activity but instead associates with phospholipid bilayers and may protect their integrity during desiccation. Pouchkina-Stantcheva et al. 5 interpret these results as divergence of a pair of alleles that have now taken on new functions - although they cannot unequivocally rule out other possibilities. They point out an unappreciated advantage of diploid asexuals: the lack of meiotic segregation permits alleles - at all gene loci simultaneously - to diverge in function in a manner that only occurs after genome duplication in sexual organisms. This potential may help to explain the paradox of the bdelloids' unprecedented adaptive radiation. It cannot be explained by this advantage alone, however, because many other microinvertebrates have clonal lineages that are diploid (for example, Daphnia, aphids and other types of rotifer), yet no similar adaptive radiation is known to have occurred in these taxa.

A second relatively recent discovery is the surprisingly high genetic diversity and efficiency of selection in bdelloids. Birky et al. 7 measured the rate of divergence among mitochondrial genes in a sample of independently evolving bdelloid clades. Mitochondrial and nuclear genes are completely linked in clonally reproducing organisms, so both have the same N_e. Birky et al. 7 found levels of nucleotide diversity to be about 1% - a value similar to most sexual species. They also compared DNA divergence rates at synonymous and nonsynonymous sites (the latter are those at which there are amino-acid substitutions) and found no evidence for weaker purifying selection in the asexual bdelloid rotifers compared with the sexual monogonant rotifers (the bdelloids' closest sexual relatives). Using this information, Dolgin and Charlesworth 8 estimated N_e for these bdelloids to be 10⁵-10⁷, assuming a range of possible mitochondrial mutation rates per base pair of 10^-9-10^-7. For comparison, the N_e for humans is 10⁴.

Is this estimated N_e of bdelloid rotifers compatible with their antiquity and taxonomic diversification? Persistence and diversification of a lineage lacking meiosis is only hindered when the Hill-Robertson effect causes selection to be overwhelmed by genetic drift (Box 1). Using the extensive human HapMap dataset 9, it was estimated that only about 20% of nonsynonymous mutations in humans will contribute to decay (via drift overpowering selection) in our gene pool 1011. As bdelloids appear to have an N_e at least an order of magnitude greater than humans, their adaptive evolution is far less constrained than ours, and is more than sufficient to permit them to persist and give rise to an adaptive radiation.

To succeed, bdelloids must be able to avoid competitive displacement by other microinvertebrates. Paland and Lynch 12 compared values of K_a/K_s (ratio of nonsynonymous mutations (K_a) to synonymous mutations (K_s)) among sexual and clonal lineages of the microinvertebrate Daphnia. From this analysis they estimated that the clonal lineages were accumulating deleterious amino-acid substitutions at four times the rate of the sexual lineages, indicating that N_e was substantially smaller in the asexual lineages, and, therefore, that meiosis plays an important role in reducing mutational load. Barraclough et al. 13 extended this approach to bdelloids, finding evidence for weaker selection on nonsynonymous codons in bdelloids compared with sexual rotifers. These two studies indicate that lack of meiosis does have a substantial cost, and this may explain why most microinvertebrates have not produced clonal adaptive radiations. Bdelloids, however, have no close sexual relatives. If early in their evolution asexual bdelloids fortuitously eliminated all close sexual relatives, and if sexual competitors have been too poorly adapted to the bdelloids' ecological niche to compete with them, then the bdelloids' large N_e (despite an elevated Hill-Robertson effect) is compatible with their observed persistence and adaptive radiation.

Why is the N_e of ameiotic bdelloids so large? Onefactor is the enormous census size of this microinvertebrate, but another less obvious factor may be their use of anhydrobiosis to accommodate to the periodic desiccation of their environment. Anhydrobiosis is a non-metabolizing state of life induced by extreme dehydration that enables the organism to survive such conditions, and bdelloid rotifers share this adaptation with many other small animals, and with some plant seeds and bacteria. Studies in such bacteria have shown that anhydrobiosis is associated with the production of numerous double-strand-breaks (DSBs) in the DNA, with a longer duration of anhydrobiosis leading to more numerous DSBs. In response, the bacteria have evolved an increased capacity for DSB repair. Interestingly, these adaptations to anhydrobiosis have fortuitously produced high resistance (more than 100 times normal levels) to ionizing radiation, which also produces DSBs 14. Although detailed molecular studies are lacking, the fact that anhydrobiotic microinvertebrates such as tardigrades 15 and bdelloid rotifers (as briefly described in 16) are also highly resistant to ionizing radiation indicates that they too are capable of repairing the numerous DSBs that would be generated during anhydrobiosis.

From bacteria to vertebrates DSBs are repaired primarily by a pathway that includes genetic exchange (via formation of heteroduplex DNA and gene conversion) between homologous chromosomes 17. An episode of anhydrobiosis in bdelloids is expected to lead to many DSBs and hence to many mitotic gene-conversion events that will sometimes lead to mitotic crossing-over. Bdelloids may be unusually susceptible to DSBs during anhydrobiosis because, unlike most other organisms known to be capable of anhydrobiosis, they do not produce the disaccharide trehalose, which promotes tolerance to desiccation 18. Because of anhydrobiosis, bdelloid rotifers are expected to have a limited form of 'selfing' due to far higher levels of mitotic recombination than in organisms lacking anhydrobiosis. Such mitotic recombination must be rare, however, as pairs of alleles with nearly identical sequences have not yet been found in bdelloids.

On a more speculative note, the ecology of bdelloids may also cause them to experience limited amounts of out-crossing via transformation. Bdelloid rotifers are filter feeders that consume substantial amounts of dead or decaying material. As a consequence, DNA will be transferred between dead and living individuals, and this can potentially lead to recombination via transformation, especially when one considers the degradation of cell membranes that occurs during anhydrobiosis. This possibility is supported by recent comparisons of alleles shared by different bdelloid lineages 19 in which one author concluded 20 that "some virtually identical alleles are shared between otherwise divergent species [of bdelloids], suggesting that bdelloids may have some unusual means of occasionally sharing genetic material."

Interestingly, the need to repair DSBs after anhydrobiosis will partially offset the advantage of allelic divergence reported by Pouchkina-Stantcheva et al. 5 because divergence of homologs will interfere with homology-dependent repair 17 and thus other, more error-prone, repair pathways will be used. Bdelloids could solve, and might have already solved 20, this problem on an evolutionary time scale by increasing their ploidy 19, as has occurred in anhydrobiotic bacteria, and hence generating new targets for homology-dependent DSB repair.

So do the recent studies of bdelloids indicate that they are an "evolutionary scandal"? Their high radiation tolerance 16 and the sharing of nearly identical alleles between otherwise distantly related lineages 20 indicate that bdelloids are not devoid of all recombination. So the scandal may be more gossip than fact. Nonetheless, the high divergence among presumed alleles 56 indicates that bdelloids are almost completely asexual, so their success does indicate that a life without meiosis is not precluded by the reduction in N_e due to Hill-Robertson interference, at least in abundant organisms like bdelloids that lack closely related sexual lineages. At present, our window into the bdelloid genome is limited by the small number of genes (about a half dozen) that have been sequenced. Genome projects on bdelloids and their sexual relatives are critically needed to provide broader comparisons for evaluating the scandal status of this unique taxon.

The cost of the Hill-Robertson effect in asexuals can be expressed as a reduced effective population size (N_e, the size of an idealized, random-mating population with only chance fluctuations in family sizes) compared with the actual population size (census size, N). In natural populations, allele frequencies change deterministically owing to selection, and stochastically owing to genetic drift. The capacity for selection to be effectual despite drift is quantified by the simple metric Sel_w/drift = 4N_e|s| 21, where Sel_w/drift is an index of the efficacy of selection after accounting for drift, and s is the selection coefficient of a mutation (for example, if a mutation decreases fitness by 1% then s = -0.01). To a close approximation, when Sel_w/drift < 1, drift overpowers selection, making it ineffectual, and when Sel_w/drift > 1, selection overpowers drift. Smaller N_e increases the contribution of drift relative to selection, so the reduction in the efficacy of selection due to asexuality (the Hill-Robertson effect) can be expressed as a reduction in N_e. Meiosis shuffles nonallelic polymorphisms each generation and thereby increases the efficiency of natural selection by breaking down interfering haplotypes, which persist in ameiotic species like bdelloids. The Hill-Robertson effect can be generated by large-effect mutations that are deleterious (background selection) or beneficial (selective sweeps) or by a mixture of favored and disfavored mutations with small effects (Hill-Robertson interference). See also Figure 1.