This fungus has two mating-types/sexes, and mating typically requires two individuals with opposite mating types. It had been a mystery why one mating type is overwhelmingly predominant in nature and how the capacity for sexual reproduction is maintained in a largely unisex population.

In an article published in PLoS Genetics, Xiaorong Lin and colleagues from Duke University Medical Center analyzed natural Cryptococcus diploid hybrid isolates containing two alleles of different serotypic origins. Natural AD hybrids that arose by fusion between two cells of different serotypes (A and D) were identified and characterized, demonstrating that same-sex mating transpires in nature. Same-sex mating had been observed under laboratory conditions, but direct evidence for naturally occurring same-sex mating was lacking. This study provides the first evidence of this and sheds light on the genetic and environmental factors that play important roles in the evolution of the current population structure of this pathogenic fungus.

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Because these tests trace only one bloodline, however, they exclude most ancestors. Moreover, they cannot pinpoint where these ancestors lived. "Each test examines less that one percent of the test-taker's DNA and sheds light on only one ancestor each generation," the study says.

A third option, known as AncestryByDNA, or admixture testing, is more promising in that it examines non-sex chromosomes inherited from both parents, chromosomes that contain DNA segments from all ancestors. To a limited extent, this test can track the geographical movements of ancestors by examining single nucleotide polymorphisms (SNPs), some of which influence such traits as skin color and resistance to regional diseases. That said, the same SNPs may be found among several populations around the world, and thus can produce false leads.

"Worldwide patterns of human genetic diversity are weakly correlated with racial and ethnic categories because both are partially correlated with geography," the study says.

Moreover, the success of genetic matching depends largely on the number of samples in a company's database. "Even databases with 10,000 to 20,000 samples may fail to capture the full array of human genetic diversity in a particular population or region," the study says.

Furthermore, the study says, AncestryByDNA tests rely on "ancestry informative markers" (AIMs), which show genetic differences between what are assumed to be four biologically distinct populations: Africans, Europeans, East Asians and Native Americans.

But "the AIMs that characterize 'Africans,' for example, were chosen on the basis of a sample of West Africans. Dark-skinned East Africans might be omitted from the AIMS reference panel of 'Africans' because they exhibit different gene variants," the study points out.

The AncestryByDNA test also reads certain markers found in people from the Middle East, India and the Mediterranean region to be diagnostic of Native American ancestry, for which there is no historical, archeological or genetic evidence, according to the study.

Indeed, the article gives very little credence to these tests, which it concludes "cannot pinpoint the place of origin or social affiliation of even one ancestor with exact certainty."

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