B Chromosome Polymorphism in Maize Landraces: Adaptivevs. Demographic Hypothesis of Clinal Variation

Abstract

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 ½simple 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.

Genome size variation in wild and cultivated maize along altitudinal gradients

“…Nonetheless, the pattern of observations is superficially consistent with a limit on maize GS as a function of elevation or variables correlated with elevation. We note that such limits on GS in maize have been argued previously based on the abundance of B chromosomes at different altitudes (Lia et al, 2007). …”

Evaluation of Cross-Species Transferability of SSR Markers in Foeniculum vulgare

“…In our study, the sequencing and alignment of fennel and carrot SSR loci revealed a general low degree of homology between the flanking regions and the repetitive motifs, and this could explain the observed low rates of transferability. Similar patterns of mutation, in terms of number of SSR repeat units, base substitution, and insertions/deletions (indels) within and outside the microsatellite motif were reported by other authors [68,69]. On the other hand, due to the lack of an EST-SSR bibliography in species closely related to fennel, we used genomic microsatellites known for their high polymorphism but low transferability, being slightly conserved.…”

Evolution of Plant B Chromosome Enriched Sequences

“…[7,8,98]. Indeed, there are several cases of B polymorphisms whether numerical or structural [99,100]. For the B chromosome of the grasshopper E. plorans a large variety of structural variants has been demonstrated among many populations [99].…”