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Abstract Three cross-incompatibility loci each control a distinct reproductive barrier in both domesticated maize (Zea mays ssp. mays) and its wild teosinte relatives. These 3 loci, Teosinte crossing barrier1 (Tcb1), Gametophytic factor1 (Ga1), and Ga2, each play a key role in preventing hybridization between incompatible populations and are proposed to maintain the barrier between domesticated and wild subspecies. Each locus encodes both a silk-active and a matching pollen-active pectin methylesterase (PMEs). To investigate the diversity and molecular evolution of these gametophytic factor loci, we identified existing and improved models of the responsible genes in a new genome assembly of maize line P8860 that contains active versions of all 3 loci. We then examined 52 assembled genomes from 17 species to classify haplotype diversity and identify sites under diversifying selection during the evolution of these genes. We show that Ga2, the oldest of these 3 loci, was duplicated to form Ga1 at least 12 million years ago. Tcb1, the youngest locus, arose as a duplicate of Ga1 before or around the time of diversification of the Zea genus. We find evidence of positive selection during evolution of the functional genes at an active site in the pollen-expressed PME and predicted surface sites in both the silk- and pollen-expressed PMEs. The most common allele at the Ga1 locus is a conserved ga1 allele (ga1-Off), which is specific haplotype containing 3 full-length PME gene copies, all of which are noncoding due to conserved stop codons and are between 610 thousand and 1.5 million years old. We show that the ga1-Off allele is associated with and likely generates 24-nt siRNAs in developing pollen-producing tissue, and these siRNAs map to functional Ga1 alleles. In previously published crosses, the ga1-Off allele was associated with reduced function of the typically dominant functional alleles for the Ga1 and Tcb1 barriers. Taken together, this seems to be an example of an allele at a reproductive barrier locus being associated with an as yet undetermined mechanism capable of silencing the reproductive barrier. 

ABSTRACT AimStudies assessing large‐scale patterns of microbial diversity have predominantly focused on free‐living microorganisms, often failing to link observed patterns to established theories regarding the maintenance of global diversity patterns. We aimed to determine whether foliar fungi on two closely related grass hosts—Heteropogon contortusandThemeda triandra—display a commonly observed latitudinal gradient in species richness and determine whether host identity, energy (temperature and precipitation), climate seasonality, fire frequency and grass evolutionary history drive the observed patterns in species richness and composition. LocationPaleotropical. Time PeriodContemporary. Major Taxa StudiedFoliar fungi. MethodsFoliar fungal diversity was quantified from 201 leaf samples ofT. triandraandH. contortuscollected across the distributional range of these species. Mixed effects models were used to quantify patterns of diversity and their correlates among and within continents. Ordinations were used to assess drivers of composition. ResultsFoliar fungi displayed consistent latitudinal diversity gradients in richness. Energy was a strong driver of richness at inter‐continental and continental scales, while other factors had inconsistent impacts on richness among scales, hosts and guilds. Globally, richness was higher in regions of higher growing season temperatures and where hosts were present for longer periods. Composition was primarily structured by geographic region at the global scale, indicating that distance was a dominant driver of community composition. Within Australia, temperature and rainfall seasonality and the amount of growing season rainfall, were the dominant drivers of both richness and composition. Main ConclusionsWe find some support for the idea that foliar fungal species diversity is governed by the same factors as many macro‐organisms (energy availability and evolutionary history) at inter‐continental scales, but also that fungal diversity and composition in the highly seasonal continent of Australia were driven by factors that shape tropical grassy ecosystems, namely climate seasonality and fire.