Poaceae

A grass is a flowering plant in the family Poaceae, which includes maize, rice, wheat and bamboos. Typical grass leaves have a sheath clasping a jointed plant stem and a blade extending away from it; a ligule often occurs at their junction. The flowers are grouped into spikelets, and the characteristic fruit is a caryopsis.^[ref]

Recognizing the family requires more than seeing narrow green leaves. Many grass stems have hollow intervals between solid joints, but maize has substantial internal tissue and bamboos can be woody. Shared floral organization provides a more reliable basis for comparison than one vegetative feature. Grass biology connects the structure of a lawn to the grain crops that feed people.

Triticum is the taxonomic genus containing wheat, within the grass family Poaceae. A taxonomic genus groups related species; it is consequently broader than Triticum aestivum, the species used as common bread wheat. Keeping these ranks separate prevents a result measured in one cultivated species from automatically becoming a statement about every Triticum.^[ref]

The genus is especially useful for studying polyploidy. In bread wheat, three related ancestral genomes are represented twice, conventionally written AABBDD. Seven chromosomes per ancestral set give $2\times (7+7+7)=42$ chromosomes. The letters label related inherited sets; they do not mean that there are only three chromosomes.^[ref]

During meiosis, a chromosome must distinguish its homologous chromosome from similar chromosomes belonging to the other ancestral sets. Comparing chromosome pairing in ordinary wheat with pairing after alteration of the Ph1 locus tests how this distinction is controlled. Thus Triticum connects crop classification with the cell-level problem of distributing multiple related genomes reliably between generations.^[ref]

Zea is a taxonomic genus of grasses that includes maize and its wild relatives, commonly called teosinte. Its native distribution is in Mexico and Central America; widespread cultivated and introduced populations should be distinguished from that native range.^[ref]

The plants exhibit monoecy: male and female flowers occur on the same plant, in separate inflorescences. The male inflorescence is terminal, while the female inflorescences are borne in leaf axils. In maize, the conspicuous male structure is the tassel, and the female structure develops into the ear. Thus an ear and a tassel are different reproductive structures of one plant.^[ref]

Comparing maize with teosinte makes Zea a powerful system for studying domestication. Differences in branching can arise through changes in gene regulation: work on the teosinte branched1 gene connects the compact architecture of cultivated maize with altered control of shoot development.^[ref]

Wheat refers to cereal grasses of the taxonomic genus Triticum, cultivated for their grain. Bread wheat, Triticum aestivum, exhibits allopolyploidy: its somatic cells contain six chromosome sets, conventionally written AABBDD, totaling $2n=42$. The letters distinguish three related ancestral genomes, each represented twice.^[ref]

This ancestry creates a problem during meiosis. A chromosome must normally pair and exchange DNA with its true homologous chromosome, rather than with a related chromosome from a different ancestral genome. The Ph1 locus helps stabilize that choice. In experiments comparing wheat with and without this locus, researchers used chromosome imaging and markers of chromosome synapsis and chromosomal crossover. Loss of Ph1 delayed synapsis relative to the stage at which chromosome ends cluster, allowing more opportunity for pairing between related chromosomes. The experiments also changed growth conditions and measured resulting crossovers.^[ref]

Polyploidy therefore involves more than carrying extra DNA: successful reproduction requires mechanisms that organize related chromosomes into reliably inherited sets.