Polyploidy in legumes

Abstract

Legumes are the third largest family of flowering plants, with over 700 genera and more than 19,000 species. Genomic evidence has shown that a whole-genome duplication (WGD) occurred shortly after the origin of the family, in an ancestor that gave rise to the papilionoids, the clade that comprises 65 % of the genera and 71 % of the species, including nearly all of the economically important crop legumes. This polyploidy event may have been associated with the origin of nitrogen-fixing symbiosis (nodulation) in the papilionoids. Nodulation most likely evolved independently in other legumes outside the papilionoids, hence there appears to be no requirement for polyploidy in the evolution of this important symbiosis. More recent polyploidy, as inferred from chromosome counts, occurs in approximately a quarter of all legume genera for which data are available. In most cases, polyploidy is confined to individual genera, species within genera, or cytotypes within species. An exception is the core clade of the genistoid legumes, a major papilionoid group that includes lupines (Lupinus). This group is probably fundamentally polyploid and also has a propensity for further polyploidy and aneuploidy in many of its genera. The frequency of polyploidy varies considerably among clades of the family, being most common (outside the genistoids) in the largely temperate, herbaceous Hologalegina (including pea and clover), and low in woody tropical groups such as the caesalpinioids.