The Landscape of Maize in Southern Mexico: Present and Future Patterns of Diversity.

In centers of crop origin, crop diversity has evolved in response to human-mediated and environmental forces for thousands of years resulting in a spectacular diversity of landraces arrayed across the landscape.  These patterns of phenotypic and molecular diversity hold clues to the crop’s evolutionary past and keys for its future. To illustrate this we discuss maize diversity in the southern state of Chiapas, Mexico, where more than 20 racial types of maize are reported in germplasm banks, three of which (Oloton, Comiteco and Tuxpeño) occupy more than 80% of the maize acreage.  Many factors affect the racial, molecular genetic, and adaptive diversity found in Chiapas.  Recent analysis comparing germplasm bank collections has clarified patterns of expansion of some races and reduction or loss of others.  For example, the expansion of improved varieties of Tuxpeño threatens minor races as Tuxpeños interbreed with local types.  Although we have shown that ethnolinguistic groups play a role in organizing maize diversity, the environment (especially temperature) continues to be a main driver for the distribution of races.  As such, adaptive differentiation is more common than differentiation at neutral molecular markers, especially when concerning traits such as days to flowering and kernel color.  Our field research has shown that Oloton is narrowly adapted to cooler highland conditions and promises to be the most sensitive to the warmer conditions predicted with climate change.  This complex balance of farmer-evolved landraces, modern cultivars, and hybridized forms promises to shift over the next decade as social and environmental changes continue.  We would expect farmers in mid and high elevations to maintain landrace production, but there is a need to more generally promote, study, and maintain the use of maize diversity on the landscape into the future.  As scientists, we should work In centers of crop origin, crop diversity has evolved in response to human-mediated and environmental forces for thousands of years resulting in a spectacular diversity of landraces arrayed across the landscape.  These patterns of phenotypic and molecular diversity hold clues to the crop’s evolutionary past and keys for its future. To illustrate this we discuss maize diversity in the southern state of Chiapas, Mexico, where more than 20 racial types of maize are reported in germplasm banks, three of which (Oloton, Comiteco and Tuxpeño) occupy more than 80% of the maize acreage.  Many factors affect the racial, molecular genetic, and adaptive diversity found in Chiapas.  Recent analysis comparing germplasm bank collections has clarified patterns of expansion of some races and reduction or loss of others.  For example, the expansion of improved varieties of Tuxpeño threatens minor races as Tuxpeños interbreed with local types.  Although we have shown that ethnolinguistic groups play a role in organizing maize diversity, the environment (especially temperature) continues to be a main driver for the distribution of races.  As such, adaptive differentiation is more common than differentiation at neutral molecular markers, especially when concerning traits such as days to flowering and kernel color.  Our field research has shown that Oloton is narrowly adapted to cooler highland conditions and promises to be the most sensitive to the warmer conditions predicted with climate change.  This complex balance of farmer-evolved landraces, modern cultivars, and hybridized forms promises to shift over the next decade as social and environmental changes continue.  We would expect farmers in mid and high elevations to maintain landrace production, but there is a need to more generally promote, study, and maintain the use of maize diversity on the landscape into the future.  As scientists, we should work to maintain farmer’s participation in crop evolution.