Symbiont-induced thelytoky and adaptation in the mite Brevipalpus phoenicis, a tropical pest

There is no doubt that sex is the major reproductive system in all walks of life, with obligate asexuality comprising little more than 1 in every 1000 of all named animal species. Most asexual organisms are known to have arisen recently from close sexual relatives and represent short evolutionary branches on the tree of life. Thelytoky (female parthenogenesis) is often thought to be an evolutionary dead end compared to sexual reproduction, because it increases homozygosity and mutational load and limits or inhibits genetic recombination. Because of this, thelytokous species are believed to lack genetic flexibility or ability to respond to environmental changes, and to suffer from the expression of recessive deleterious mutations. Yet there are also important higher taxa made up entirely of asexual (thelytokous) parthenogens including the order Bdelloidea of rotifers and the tribe Tramini of aphids. The Acari (mites) also contain various families that are wholly thelytokous. Despite popular opinion, this suggests that some thelytokous parthenogens can be evolutionary successful. An example is the thelytokous false spider mite Brevipalpus phoenicis. This species is unique. It is the only known example with haploid thelytoky, and we recently discovered that thelytoky is caused by cytoplasmic bacteria. But yet, it is extremely polyphagous and widely distributed in the tropics and sub-tropics. In addition, it is a major pest on citrus in Brazil and tea in Indonesia, because it develops resistance to acaricides rapidly and vectors citrus leprosis virus. The extreme adaptability and thelytokous mode of reproduction present a paradox: adaptation requires genetic variation, but thelytoky restricts the generation of genetic variation. One way out of this paradox is the occasional reversal to a sexual mode of reproduction and allow for recombination and generation of new clonal variants. This may be achieved by hybridization with males of sexual forms or males produced upon loss of bacterial infection. An alternative, but not mutually exclusive model is the "general purpose genotype" model. This model argues that selection among an array of (continuously generated) clones will result in the persistence of only those clones with broad ecological tolerance. The current project aims at investigating the origin of clonal genetic variation, their width of tolerance towards host plant species and abiotic factors (including pesticide), and at assessing variation in the mode of reproduction in B. phoenicis.