Multiple origins of hybrid strains of Cryptococcus neoformans with serotype AD

Cryptococcus neoformans is a major pathogen of humans throughout the world. Using commercial mAbs to capsular epitopes, strains of C. neoformans manifest five distinct serotypes--A, B, C, D and AD. Previous studies demonstrated significant divergence among serotypes A, B, C and D, which are thought to be haploid. In this study the origins and evolution of strains of serotype AD were investigated. A portion (537 bp) of the laccase gene was cloned and sequenced from 14 strains of serotype AD. Each strain contained two different alleles and sequences for both alleles were obtained. These sequences were compared to those from serotypes A, B, C and D. This analysis indicated that each of the 14 serotype AD strains contained two phylogenetically distinct haplotypes: one haplotype was highly similar to the serotype A group and the other to the serotype D group. To explain the origins of these serotype AD strains, genealogical analysis is consistent with at least three recent and independent hybridization events. The results demonstrate that the evolution of C. neoformans is continuing and dynamic.