Subsequent to the writing of this Perspective, Ketel et al. isolated a fourth distal neocentromere, derived after growth on sorbose medium from the same YJB9929 strain that had already acquired two separate neocentromere positions. As the authors point out, this observation clearly indicates that the position of a neocentromere can spontaneously shift large distances (at least 40kb) in C. albicans.

Such an observation of neocentromere mobility has not been previously reported in the literature, and it may be unique to C. albicans neocentromeres. However, the factors influencing such a large positional shift remain uncertain, as neocentromere mobility in C. albicans was only observed within one transformant strain isolated in the study. In contrast, another isolated strain that had formed a neocentromere near the telomere of Chr5 remained stable (with some degree of local positional shift), as did all proximal neocentromeres examined. The identification and classification of more transformant strains with distal neocentromeres should elucidate whether such positional variation within C. albicans neocentromeres is common.

Although Ketel et al. failed to identify any common DNA sequence elements within the CENP-A binding domains of the neocentromeres themselves, further analysis of the regions surrounding the CENP-A binding domains demonstrated that all four distal neocentromeres were closely associated with inverted or direct repeats. The similarity with the native centromeres of C. albicans (where the centromeric regions are also flanked by repeats on all but one chromosome) is striking, although the functional significance of flanking repeats in centromere formation is currently unclear. Furthermore, three of the four neocentromeres were situated within close proximity to an LTR sequence. Such an observation is particularly interesting in light of recent work suggesting a link between Line1 retrotransposons and human neocentromeres (Chueh, et al. 2009), and may point to a conserved evolutionary role for retroelements in the formation of new centromeres.

References
Chueh AC, Northrop EL, Brettingham-Moore KH, Choo KHA, Wong LH (2009) LINE retrotransposon RNA is an essential structural and functional epigenetic component of a core neocentromeric chromatin. PLoS Genet 5:e1000354
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