Chimeras for Genetic Analysis

Abstract

A chimera—defined as an individual (plant) composed of two or more genotypes— usually results from any heritable change that provides different expression by the descendants of two daughter cells from a mitotic cell division. Chimeras can be very useful in revealing the action of mutant genes in plants. McClintock (1938) first demonstrated the feasibility of using deficiencies and unstable ring chromosomes to study the consequences of the absence of functional genes in homozygous mutant tissue. Stadler and Roman (1948) identified three previously unknown genes near the a1 locus by uncovering different lengths of X-ray induced deficiencies (a-x1, a-x2, a-x3) with an unstable fragment (A-b Frag) carrying functional genes located in the deficiency segments. Similar studies by McClintock (1951, 1965), Steffensen (1968), Coe and Neuffer (1978), and Johri and Coe (1983) demonstrate the utility of this approach in solving problems in biology. An advantage of chimeras in genetic research is that they provide mutant tissue that is of identical age and supported by adjoining normal tissue; this allows expression of lethal mutants that would not normally be observable. Chimeras may arise spontaneously for unspecified reasons or they may be produced by unstable chromosome configurations (inversions, rings, centric fragments, monosomics), by chromosome breaking agents (radiation or chemicals) or by unstable loci resulting from transposon insertion (Ds, etc.). Analysis of chimeras can tell us that some genes encode phenotypes that are cell autonomous (anthocyanin and chlorophyll genes) and that some are not (andromonoecious dwarfing). They also (1) show that some lethal mutants may be rescued by supplying missing chemicals, (2) indicate the developmental pattern of the plant parts, and (3) provide clues as to the sequence of events in a biosynthetic pathway.