Genetic incompatibilities among populations of cassava green mite complex mononychellus spp. (acari:tetranychidae) and their implication in the taxonomy of the mite

Abstract

Hybridization tests revealed various levels of reproductive isolation among 19 populations tested. Mean egg lethality was highest in the F2 than in the F1 and F3 (F1<F2>F3; p<0.0003).Populations from Nyanza, Western and Coast had higher lethalities compared to those from Central Kenya (mean lethality =11.69, 9.26 and 6.35%, respectively). Sub-sp~cific populationaldifferences were recorded on populations from Central Kenya (p<0.05) compared to other sites (p=>0.05). However intra-populational check crosses revealed that these differences were not due to gene interaction but resulted from heterogeneity as a result of extra-chromosomal factors (p<0.05) in the different setal morphs. This is evidenced by successful zygote formation and therefore, a conunon gene pool is shared. Hybrid success of the susequent progeniesindicated absence of hybrid sterility, inviability and, or breakdown and distorted sex-ratios (range=l:l-8:1). Preponderance of diploid offspring indicated fertilization was successful among all the populations . hybridized. The arrhenotokous mode of reproductive parthenogenesis was demonstrated which gave an exclusive haploid male progeny from uninseminated virgin females ..Based on the shape of their aedeagii, all the males from the six sites were identified as Mononychellus progresivus while the 6 females were classified as short, long or intermediate setaed parents (range= 20.02-45.76 μm \). The 6 F1 generation lines crossed segregated into distinguishable short, long and intermediate setal forms (range=l7.56-33.24 '.~m). The D1, D2 and D3 setae increased with body size (r=0.877, 0.97 and 0.93, respectively; p<0.0001), but were different in each of the 6 sites (p<0.0001). The existence of the three setal morphs suggested that setae inheritance is polygenic and is controlled by three non-allelic genes. Because of their great variability, the dorsal setae caunot, therefore, be reliably used for species diagnosis. The shape of the aedeagus was found to be the only reliable morphological species diagnostic tool due to its genetic stability. It is questioned whether M. tanajoa and M. progresivus are discreet species. The weight of the evidence justifies the conclusion that M. tanajoa and M. progresivus are one and the same species.