Variance in mtDNA Among Bantu-speaking Populations

A study of mitochondrial DNA in two large Bantu-speaking groups, the Shona and the Hutu, shows that both groups display nearly all of the haplogroup markers characteristic of Bantu-speakers. Dispersal Bantu-speaking peoples in Africa was likely a gradual process, as evidenced by the pattern of genetic variability shown in this paper. However, some notable differences in mtDNA haplogroup composition seems to suggest that there was continued gene flow between already dispersed Bantu-speaking groups, as well as between Bantu and non-Bantu speaking groups. Interestingly, such variability was not found on the Y chromosome. Might this be explained by marriage and settlement patterns?