B chromosome polymorphism in maize landraces: adaptive vs. demographic hypothesis of clinal variation.

Cytogenetic analysis of maize landraces from northwestern Argentina has revealed an altitudinal cline in the mean number of B chromosomes (B's) per plant, with cultivars growing at higher altitudes exhibiting a higher number of B's. Altitudinal and longitudinal clines are frequently interpreted as evidence of selection, however, they can also be produced by the interplay between drift and spatially restricted gene flow or by admixture between previously isolated populations that have come into secondary contact. Here, we test the adaptive significance of the observed altitudinal gradient by comparing the levels of differentiation in the mean number of B's to those obtained from 18 selectively neutral loci [simple sequence repeats (SSRs)] among seven populations of the cline. The adequacy of alternative genetic-differentiation measures was determined, and associations between cytogenetic, genetic, and altitudinal distances were assessed by means of matrix- correspondence tests. No evidence for association between pairwise F(ST) and altitudinal distance or B-chromosome differentiation was found. The contrasting pattern of altitudinal divergence between the mean number of B's per plant and the genetic differentiation at SSR loci indicates that demographic processes cannot account for the observed levels of divergence in the mean number of B's.