Genetic and morphological characteristics of an architectural mutant in common beans were studied. The mutant had shiny, dark green leaves, overlapping leaflets, short petioles and a reproductive terminal bud even though the line did not carry the fin gene. Branching was nearly absent resulting in a single stem vine. This is a new form of determinancy in common bean. The inheritance of the mutant was studied in crosses involving four normal parents, Oregon 91G, Minuette, 5-593, and UI 906, which represent a diversity of growth habit types. The F₁ plants had a normal phenotype, indicating that the mutant trait was recessive. F₂ segregation data fit a single recessive gene ratio and F₃ data confirmed the hypothesis that the mutant character was controlled by a single recessive gene. Tests of allelism were conducted to determine if the mutant was allelic to similar mutants. These were ol for overlapping leaflets and dgs for dark green savoy leaves. When the mutant was crossed with ol and dgs tested, all of the F₁ plants were normal. These results are consistent with the interpretation that the mutant trait was not allelic to ol and dgs. The name topiary was conferred on this mutant describing its compact and neat appearance. The gene symbol top was proposed. Because the four parental lines differ from the top line for alleles at other loci, linkage was tested for growth habit (fin), shiny leaf, cross-sectional shape of pods, striped pod (prp superscript [st]) and pod suture strings (st) with the topiary mutant. Chi-square test of independence showed no linkage between either the mutant and marker genes or among the marker genes. A phenological study of topiary revealed that long days delayed flowering an average of 15 days. In addition, the number of vegetative nodes before flowering was increased. The topiary mutant is photoperiod sensitive, but does not react in the same manner as other photoperiod sensitive beans. Whether this photoperiodic sensitivity in the mutant is independent of the topiary gene is unknown and deserves further study. The topiary mutant has potential for improving common beans. Its single stem growth habit may allow closer row spacing leading to higher planting populations and may enhance the efficiency of mechanical harvest. Pod formation at higher nodes may escape disease. Currently, the thin stems cause lodging. Development of thick and upright forms will be the subject of future studies
