Stanley Trask and colleagues hypothesize that most
HIV-1 transmissions in sub-Saharan Africa occur between married couples. That is, the husband acquires HIV and then passes it to his wife, or the wife acquires
HIV and then passes it to her husband. The researchers then use a reconstructed evolutionary tree to test their hypothesis. Think about how this test might work.
What would the tree look like if the hypothesis is true? If
it is false? Then look up Trask et al.’s paper:
Trask, S. A., C. A. Derdeyn, U. Fideli, et al. 2002. Molecular epidemiology of human immunodeficiency virus type 1 transmission in a
heterosexual cohort of discordant couples in Zambia. Journal of Virology 76: 397–405.
Also see a follow-up of this topic, which found that risk
of acquiring HIV from an infected partner is higher if
the two partners share certain immune system alleles:
Dorak, M., J. Tang, J. Penman-Aguilar, et al. 2004. Transmission of
HIV-1 and HLA-B allele-sharing within serodiscordant heterosexual
Zambian couples. Lancet 363: 2137–2139.