Demonstration of local adaptation of maize landraces by reciprocal transplantation

Abstract

Populations are locally adapted when they exhibit higher fitness than foreign populations in their native habitat. Maize landrace adaptations to highland and lowland conditions are of interest to researchers and breeders. To determine the prevalence and strength of local adaptation in maize landraces, we performed a reciprocal transplant experiment across an elevational gradient in Mexico. We grew 120 landraces, grouped into four populations (Mexican Highland, Mexican Lowland, South American Highland, South American Lowland), in Mexican highland and lowland common gardens and collected phenotypes relevant to fitness, as well as reported highland-adaptive traits such as anthocyanin pigmentation and macrohair density. 67k DArTseq markers were generated from field specimens to allow comparison between phenotypic patterns and population genetic structure. We found phenotypic patterns consistent with local adaptation, though these patterns differ between the Mexican and South American populations. While population genetic structure largely recapitulates drift during post-domestication dispersal, landrace phenotypes reflect adaptations to native elevation. Quantitative trait QST was greater than neutral FST for many traits, signaling divergent directional selection between pairs of populations. All populations exhibited higher fitness metric values when grown at their native elevation, and Mexican landraces had higher fitness than South American landraces when grown in our Mexican sites. Highland populations expressed generally higher anthocyanin pigmentation than lowland populations, and more so in the highland site than in the lowland site. Macrohair density was largely non-plastic, and Mexican landraces and highland landraces were generally more pilose. Analysis of δ13C indicated that lowland populations may have lower WUE. Each population demonstrated garden-specific correlations between highland trait expression and fitness, with stronger positive correlations in the highland site. These results give substance to the long-held presumption of local adaptation of New World maize landraces to elevation and other environmental variables across North and South America.

A B73×Palomero Toluqueño mapping population reveals local adaptation in Mexican highland maize

“…The flowering QTL qDTA8, qDTS8, and qASI8 overlap a ∼10-Mb region that contains the two well-characterized flowering genes ZmRap2.7 and Zcn8 . This region and/or these genes have been reproducibly detected in linkage- and association-mapping studies of maize flowering time ( Chardon et al 2004 ; Buckler et al 2009 ; Steinhoff et al 2012 ; Li et al 2016 ; Romero Navarro et al 2017 ), temperate adaptation ( Ducrocq et al 2008 ; Bouchet et al 2013 ; Guo et al 2018 ; Castelletti et al 2020 ), and adaptation to the Mexican Highlands ( Gates et al 2019 ; Janzen et al 2021 ; Wang et al 2021 ). An early flowering Vgt1 allele from northern germplasm has previously been associated with a miniature transposon (MITE) insertion upstream of ZmRap2.7 , although the absence of the MITE alone did not explain late flowering Vgt1 alleles ( Buckler et al 2009 ).…”

“…Typically, they are cultivated under low-input conditions and produce a modest but stable yield ( Zeven 1998 ; Breseghello and Coelho 2013 ; Dwivedi et al 2016 ). The sustained association of a given landrace population with a given locality results in local adaptation, in the same way it is seen in wild populations, demonstrable by reciprocal transplantation ( Janzen et al 2021 ).…”

A B73 x Palomero Toluqueño mapping population reveals local adaptation in Mexican highland maize

“…The flowering QTL qDTA8, qDTS8 and qASI8 overlap a ~10Mb region that contains the two well-characterized flowering genes Vgt1 and Zcn8. This region and/or these genes have been reproducibly detected in linkage-and association-mapping studies of maize flowering time (Chardon et al, 2004;Buckler et al, 2009;Steinhoff et al, 2012;Li et al, 2016;Romero Navarro et al, 2017), temperate adaptation (Ducrocq et al, 2008;Bouchet et al, 2013;Guo et al, 2018;Castelletti et al, 2020) and adaptation to the Mexican Highlands (Gates et al, 2019;Janzen et al, 2021;Wang et al, 2021). An early flowering vgt1 allele from northern germplasm has previously been associated with a miniature transposon (MITE) insertion, although the absence of the MITE alone did not explain late flowering vgt1 alleles (Buckler et al, 2009).…”

“…Further sequencing of b1 alleles from highland maize will shed greater light on patterns of diversity and the origin of different alleles. Stem pigmentation, unlike stem pubescence, is also shared with South American highland maize (Janzen et al, 2021). Dark red pigmentation in the stem can help the plant to absorb more solar radiation and keep the plant warmer in a cold environment and might also protect DNA from damage due to higher UV-B radiation in the highlands (Barthakur, 1974;Eagles & Lothrop, 1994;Casati & Walbot, 2005) -although it is unclear why such protection might be required more so in the stem than in the leaf blades.…”

“…Typically, they are cultivated under low-input conditions and produce a modest but stable yield (Zeven, 1998;Breseghello & Coelho, 2013;Dwivedi et al, 2016). The sustained association of a given landrace population with a given locality results in local adaptation, in the same way it is seen in wild populations, demonstrable by reciprocal transplantation (Janzen et al, 2021).…”