The Morphology of Maize and Teosinte
John Doebley
Laboratory of Genetics
University of Wisconsin-Madison
© John Doebley 2003
 

Much of the interest and controversy surrounding the domestication of maize arises from the fact that these two plants have such strikingly different morphologies. On this page, I will describe the differences in plant and inflorescence morphology between maize and teosinte following the system that my lab has used in its QTL studies. The differences are complex and there is no substitute for actually sitting down at a microscope with the plants in hand. Some useful publications on maize and teosinte morphology are listed below.

Plant Architecture: Teosinte plants have main stalks that typically bear an elongate lateral branch at most nodes (Figure 1). The lateral branches and the main stalk are both composed of a series of nodes and elongated (15 or more cm long) internodes with a leaf attached at each node.

The number of internodes (or leaves) in each lateral branch is roughly equivalent to the number in the main stalk above the point of attachment of the branch. Thus, a branch attached on the third node below the main tassel will be composed of about three internodes, while one attached at the sixth node below the tassel will have about six internodes. The leaves along the lateral branches are fully formed and composed of two parts -- a sheath that clasps around the stem and a blade that extends away from the plant. The leaves on the branches are arranged in an alternate phyllotaxy, i.e. each leaf is borne on the opposite side of the stem relative to the leaves at the nodes above and below. Both the main stem and the primary lateral branches of the teosinte plant are tipped by male inflorescences (tassels) while the slender female inflorescences (ears) of teosinte are borne on secondary branches in the axils of the leaves along the primary branches. Each of these female inflorescences is surrounded by a single, bladeless leaf or husk. The ears occur in clusters of 1 to 5 (or more) at each node along the branch.

The architecture of the lateral branches of the maize plant is strikingly different from that of teosinte (Fig. 1). Maize typically produces branches at only two or three of the nodes along the main stem. Axillary buds are present at some of the other nodes but they are arrested early in development. Each of the branches that are produced is composed of nodes and short internodes, averaging about 1 cm in length. Unlike teosinte, the number of internodes in the lateral branch is greater than the number in the main stalk above the point of attachment of the branch. For maize line W22, a branch attached at the fifth node below the main tassel will be composed of about 12 internodes and have 12 husk leaves. The leaves (husks) along the lateral branch are composed largely of sheath with only a small (if any) blade attached to it. The husks are arranged in a spiral phyllotaxy along the branch, rather than the alternate phyllotaxy for leaves on the main stem of the plant. Secondary branches are normally absent (aborted). Finally, the lateral branch is terminated by a female inflorescence or ear, which is tightly enclosed within the spirally arranged husks because of the failure of the internodes of the branch to fully elongate.

Figure 1

The divergent morphologies of maize and teosinte lateral branches reveal the complex set of evolutionary changes involved in the origin of maize plant architecture. These include (1) arresting the full elongation of the internodes, (2) arresting the growth of the leaf blade, (3) arresting the outgrowth of some axillary buds, (4) suppressing the outgrowth of secondary branches, (5) increasing the number of leaves (husks) formed on the branch, (6) changing leaf (husk) phyllotaxy from alternate to spiral, and (7) replacing the tassel (male inflorescence) at the tip of the lateral branch with an ear (female inflorescence). Noticeably, most of these changes involve arresting the growth of particular organs. This suggests that a common developmental process (repression of organ growth) expressed at distinct points in space or time of organogenesis produced this suite of correlated changes (see tb1 and the evolution of maize).

Figure 2

Figure 3

Inflorescence Architecture: The most dramatic differences between maize and teosinte involve the architecture of their female inflorescences (Figs. 2 and 3). The teosinte ear is composed of 5 to 10 (or more) distichously (in two ranks) arranged cupulate fruitcases. The cupule of the cupulate fruitcase is formed from the invaginated rachis internode (RA). The cupule contains a single sessile spikelet that is oriented parallel to the axis of the rachis. The outer glume (OG) of this sessile spikelet seals the opening of the cupule, thus obscuring the kernel from view. Both the rachis internode and its outer glume are highly indurated in teosinte. The cupulate fruitcases are separated from one another by abscission layers, thus enabling the fruitcases to separate (disarticulate) at maturity for dispersal.

The cob (rachis) of the maize ear, like that of its teosinte counterpart, is composed of invaginated internodes or cupules (Figs. 2 and 3). Maize cupules are arranged polystichously (in four or more ranks) around the circumference of the ear with usually 100 or more cupules in a single ear (Fig. 2). Unlike teosinte, the cupules of maize are shallow, often collapsed, and they do not envelop the kernels (Figs. 2 and 3). Maize cupules may be indurate, but the outer glumes are softer than the highly indurated glumes of the teosinte ear. In contrast to teosinte, there are two spikelets associated with each cupule, one pedicellate and the other sessile. Thus, an ear with four ranks of cupules will have eight rows of kernels (Fig. 3). The female spikelets of maize also differ from those of teosinte in that they are oriented perpendicular and not parallel to the axis of the ear. Finally, maize ears lack abscission layers as found in teosinte, so the ear remains intact at maturity.

Annotated Bibliography:

• Iltis, H. H. 2000. Homeotic sexual translocations and the origin of maize (Zea mays, Poaceae): A new look at an old problem. Economic Botany 54: 7-42. [This article has a detailed description of branch formation in teosinte including an extensive set of illustrations.]
• Weatherwax, Paul. 1935. The phylogeny of Zea mays. Amer. Midl. Naturalist. 16:1-71. [This article has some of the oldest and most detailed illustrations of teosinte and other relatives of maize.]

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