Significance and context
Haplodiploidy, a state where males are haploid and females are diploid, is seen in many insect species, but so far no organism has been found to exist exclusively in the haploid state. Weeks and colleagues report that females of a mite species exist exclusively in a haploid state. Brevipalpus phoenicis, a minute phytophagous mite found in tropical and subtropical regions, is considered to be one of the major pests of many economically important crops such as citrus, coffee and tea. The false spider mite B. phoenicis and its close relatives B. obovatus and B. californicus reproduce by parthenogenesis. In the field, B. phoenicis is found almost exclusively in the female state. Both the eggs and adult cells of these female mites contain two chromosomes, but it has been difficult to unravel whether this represents a haploid or diploid state. Weeks et al. have shown that these two chromosomes are genetically distinct and that the female mites are haploid.
Weeks et al. used cytological and genetic studies to show that B. phoenicis is a haploid female parthenogen. They found that only one of the two chromosomes contains a nuclear-organizing region (NOR). If the two chromosomes were homologous, they would carry copies of the same genes in the same locations, thus, the presence of only one NOR suggests that the chromosomes are distinct and B. phoenicis is haploid. In addition, Weeks et al. used fluorescent in situ hybridization to locate the ribosomal DNA (rDNA) in the metaphase chromosomes. The B. phoenicis 18S rDNA probe hybridizes at the tip of both sister chromatids of one chromosome in a metaphase mitotic division and no hybridization signal was seen on the second chromosome. The results showed only one carries an 18S ribosomal DNA gene. The authors also screened 45 clonal lines of B. phoenicis for genetic variation at seven polymorphic microsatellite loci and found no differences at any locus.
During the investigation large numbers of endosymbiotic bacteria were found to be associated with these mites. Weeks et al. attempted to discover whether these bacteria were involved in female haploid parthenogenesis. By sequencing bacterial 16S rDNA amplified from mites, the bacterium was identified as an endosymbiote that is also found in the tick Ixodes scapularis. To establish whether the bacteria are involved in causing parthenogenesis, B. phoenicis females were treated with the antibiotic tetracycline, which eliminated the bacteria, and were then allowed to lay eggs. Significantly more male offspring were produced after tetracycline treatment than without, suggesting that female haploid parthenogenesis is caused by bacterial infection which results in feminization of genetic males.
Brevipalpus phoenicis exist only as females in the haploid state. The two chromosomes of this species are genetically distinct. Feminization of haploid genetic males results from infection by an endosymbiotic bacterium.
This is the first report that feminization by an extra-chromosomal factor has been found outside of heterogametic reproductive systems. Weeks et al. have shown that feminization can involve bacteria other than Wolbachia - a bacterium that is known to induce feminization by blocking the formation of the androgenic gland. Studying the possible infection mechanism by which these bacteria induce feminization will shed more light on the sex-determination process of these mites.