b'LIFEHow New Plant Species Ariseuntangles the mechanismthat keeps maize32 corn, a grassdistinct from some strains of its ancestorT he creation of a new species in evolution requires a reproductive barrier. Determining the genetic and molecular nature of reproductive barriers is an age-old inquiry. New research led by Carnegies Matthew Evans, with Carnegies Yongxian Lu, Samuel Hokin, and Thomas Hartwig, plus Jerry Kermicle of the University of Wisconsin, Madison, untangles the mechanism that keeps maizecorn, a grassdistinct from some strains of its ancestor teosinte. Matt Evans led the study of species distinction. Image courtesy Robin Kempster, Carnegie Institution for ScienceSpeciation requires isolation. Sometimes isolation is facilitated by geography that divides two populations, preventing them from interbreeding until they become different species. In other instances,Silkseparation is physiological, preventing successful mating or viable offspring. pollen tubesIn plants, genetic isolation can be maintained by features that prevent the male pollen of one speciespollen grainsfrom successfully fertilizing the female pistil of another species.About 9,000 years ago, maize was domesticated from teosinte in the Balsas River Valley of Mexico. Some populations of the two grasses are compatible for breeding. But others grow in the same areas and flower at the same time but rarely produce hybrids.It was known that a cluster of genes called Tcb1-s is one of three that confers incompatibility between rarely hybridizing maize and teosinte populations. ButThis image shows the pollen tubes of pollinated maize silk. The unlike the other two, it is found almost exclusively inpollen tubes were stained using aniline blue (scale bar = 100m). Image reproduced with permission of CAB International, from Maize Kernel Development'