COMPARATIVE EVOLUTION OF CEREALS.

Abstract

First, we shall introduce the materials with which we are dealing, and this requires some understanding of formal names and botanical classification. The taxonomy of cultivated plants has long been in a state of confusion. The same array of variation is treated in radically different ways by different taxonomists (see Jirasek, 1966; Jeffrey, 1968). Classifications are cluttered with Latin names that have little or no biological meaning, and some individual taxa are given ranks ranging from variety to genus depending on who is doing the classifying. Inept classifications have probably caused more difficulty in understanding the origin and evolution of cultivated plants than any other factor. We shall use the gene pool classification suggested by Harlan and de Wet (1971) in order to treat the several cereals on a uniform basis. In this system the total array of variation within maximum genetic reach is partitioned into primary, secondary, and tertiary gene pools. The primary gene pool includes all those races that can be crossed with the crop, yielding reasonably fertile hybrids in which the chromosomes pair well and in whose offspring genetic segregation is reasonably normal. The primary gene pool corresponds to the widely accepted concept of the biological species. The secondary gene pool includes all those species that can be crossed with the crop but with restricted gene flow. Genes can be transferred from the secondary to the primary gene pool, but one must struggle with those barriers that separate biological species such as sterility, poor chromosome pairing, lethal or weak hybrids, or poorly adapted hybrid derivatives and so on. The tertiary gene pool includes all those species that can be crossed with the crop, but the hybrids lead essentially nowhere. The hybrids are lethal, completely sterile, or anomalous. If any gene transfer is possible at all, it must be through radical manipulation of some sort such as embryo culture, tissue culture, use of complex hybrid bridges and so on (see Harlan and de Wet, 1971). Polyploid series in cultivated plants pose some special problems. As a general rule, we have suggested (Harlan and de Wet, 1971) that each level be treated as a separate gene pool. The barriers between ploidy levels are not necessarily strong, however, and morphological differences are sometimes minimal and difficult to describe. Each series is different and appropriate treatments must be worked out crop by crop. Separate gene pools for ploidy levels in potato or sugarcane may not be appropriate at all. In the cereals considered here, the only problem of separation by ploidy level occurs in oats where A vena strigosa and A. barbata races are difficult to distinguish morphologically. Our gene pool classification is not intended to be a formal taxonomic system but rather a simple device to bring a genetic focus to bear on the taxonomies already available. The conventional epithets can be used without undue confusion pro-