Coroico as a resource for maize improvement --Walton C. Galinat Long separated and isolated from the main pathways of corn's evolution, the race Coroico has retained at least two primitive genes, is (cupulate interspace) and tpe (thin pericarp) that are now unknown in Mexican corn, although they appear to occur in the oldest Tehuacan maize (7,200 years) and do occur in the teosintes. The is gene segregates out in the F2 of corn-teosinte hybrids as a single recessive gene. Allelism tests between is from teosinte and Coroico have not yet been made. Apparently the Guarany Indians from the interior lowlands east of the Andes in Bolivia, Peru and Brazil received and cultivated the primitive ancestor of Coroico several thousand years ago soon after its spread southward from Mexico. In their hands, Coroico evolved at least two other associated traits that are unique and of potential economic importance.

The interspace (is) factor exposes a bare face of rachis between the apex (upper lip) of the cupule and the glume cushion of the diverging spikelet(s) above. Botanically, the interspace represents the backside of the internode from the alternate rachis segment (phytomer). When condensation is sufficiently low for the length of the interspace to become equal to the length of the cupule below, then there is enough extra space to interlock the pedicellate member from an adjacent pair while the sessile member stays in line with its cupule row. The result is a 50% reduction in the kernel-row number. The most common kernel row number in Coroico is nine, on long slender ears with enlarged butts that are 18-rowed.

The exposure of the kernel from the teosinte fruitcase required that it adapt by evolving two new systems of kernel protection. An inner system of either a thickened pericarp or a thickened aleurone that would guard against "self-popping" of mature kernels due to weather and, secondly, a new outside system of husk leaves for varmint protection. The Coroico kernels have the primitive thin pericarp caused by the tpe gene of only three to four cell thickness. When expressed in a teosinte background, where apparently the tpe of Coroico originated, the pericarp is only two to three cells thick and resistant to self-popping. Viability of teosinte seed is, thereby, insured by the physical support derived from containment within the fruitcase. Protection of the exposed kernel from self-popping in Coroico under the direction of the Guarany Indians in South America has taken the aleurone pathway while in Central America the pathway was by the pericarp. Sweet corn improvement based on a tender or thin pericarp may be best achieved by use of the Coroico complex (see my item here on multi-layered expression). The tpe gene is incompletely dominant and in some backgrounds it may be inviable due to pericarp splitting and pathogen infection. The multiple aleurone factor may be inherited as a single dominant gene (Wolf et al., Crop Sci. 12:440, 1972).

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