Identification of teosinte, maize, and Tripsacum in Mesoamerica by using pollen, starch grains, and phytoliths

Abstract

We examined pollen grains and starch granules from a large number of modern populations of teosinte (wild Zea spp.), maize (Zea mays L.), and closely related grasses in the genus Tripsacum to assess their strengths and weaknesses in studying the origins and early dispersals of maize in its Mesoamerican cradle of origin. We report new diagnostic criteria and question the accuracy of others used previously by investigators to identify ancient maize where its wild ancestor, teosinte, is native. Pollen grains from teosinte overlap in size with those of maize to a much greater degree than previously reported, making the differentiation of wild and domesticated maize in palynological studies difficult. There is presently no valid method for separating maize and teosinte pollen on a morphological basis. Starch grain analysis, a recently developed tool of archaeobotany, appears to be of significant utility in distinguishing the seeds of teosinte from maize. We propose that the differences in starch grain morphology and size between wild and domesticated maize defined in this study may be associated with domestication genes in Zea that have been documented in the starch biosynthesis pathway. As previously reported, phytoliths effectively discriminate the female reproductive structures of Tripsacum, teosinte, and maize. Multiproxy microfossil studies of archaeological and paleoecological contexts appear to be effective tools for investigating the earliest stages of maize domestication and dispersals.

Starch grain and phytolith evidence for early ninth millennium B.P. maize from the Central Balsas River Valley, Mexico

“…We focused our analyses on starch grains and phytoliths, which are effective indicators of wild and domesticated maize and squash remains in archaeological contexts (6)(7)(8)(9)(10)(11)(12)(13)(14). As is common in other tropical regions (6,7,(10)(11)(12)(13), the preservation of macrofossil plant remains was very poor, consisting of wood charcoal and a few unidentified seed fragments.…”

Microscopic evidence for the domestication and spread of maize

“…However, the problem of how to reliably separate the fossil pollen of the other two taxa has remained unresolved. After a reexamination of the problems associated with maize pollen recognition and after conducting additional studies, Holst et al (1) concur that fossil pollen of Tripsacum can be separated from the other two, but that there is no reliable way to separate the fossil pollen of teosinte from those of the cultivated races of maize.…”

“…Pohl et al presented convincing phytolith evidence for the early spread of domesticated maize 7,300 years ago from its origin in Southwest Mexico to the Gulf Coast region of Tabasco. In a recent issue of PNAS, Holst et al (1) presented studies showing the reliability of these types of microscopic data in distinguishing maize from teosinte, something that cannot yet be achieved through the use of fossil pollen evidence.…”

“…During that time, there have been numerous hypotheses and assurances that these mysteries had finally been solved, only to be rejected by yet another new discovery. Now, in a recent issue of PNAS, Holst et al (1) provide us with a new set of answers to these age-old questions and present a ''tripartite method'' focused on pollen, phytoliths, and starch as a way to resolve these questions now and in the future.…”

On the Taxonomic Resolution of Pollen and Spore Records of Earth’s Vegetation

“…Continuing the example of grass pollen, phase-contrast microscopy has been employed to acquire information on the organization of the exine (Faegri et al 1992;Beug 2004), and this has been used to generate broad groupings of grass pollen morphotypes (Beug 2004) and to distinguish Tripsacum pilosum, which has clumped tectal columellae, and Zea mays ssp. parviglumis (Balsas teosinte), which has uniformly distributed tectal columellae (Holst et al 2007). Such features of the grass pollen exine cannot be viewed with an SEM because this instrument provides information only from the surface of a specimen (Sivaguru et al 2012).…”