A pistil-expressed pectin methylesterase confers cross-incompatibility between strains of Zea mays

Yongxian Lu,Mathew M S Evans,Jerry L Kermicle,Thomas Hartwig,Samuel A Hokin

doi:10.1038/s41467-019-10259-0

Abstract

A central problem in speciation is the origin and mechanisms of reproductive barriers that block gene flow between sympatric populations. Wind-pollinated plant species that flower in synchrony with one another rely on post-pollination interactions to maintain reproductive isolation. In some locations in Mexico, sympatric populations of domesticated maize and annual teosinte grow in intimate associate and flower synchronously, but rarely produce hybrids. This trait is typically conferred by a single haplotype, Teosinte crossing barrier1-s. Here, we show that the Teosinte crossing barrier1-s haplotype contains a pistil-expressed, potential speciation gene, encoding a pectin methylesterase homolog. The modification of the pollen tube cell wall by the pistil, then, is likely a key mechanism for pollen rejection in Zea and may represent a general mechanism for reproductive isolation in grasses.

Highlights

A central problem in speciation is the origin and mechanisms of reproductive barriers that block gene flow between sympatric populations
While Gametophyte factor1-s (Ga1-s) and Gametophyte factor2-s (Ga2-s) are widespread in domesticated maize, Teosinte crossing barrier1-s (Tcb1s)-s is almost exclusively found in wild teosinte populations, with the exception of one ancient Maiz Dulce sweet corn variety from Mexico[8]
There are two alternative explanations of rejection of tcb[1] pollen: (1) tcb[1] pollen expresses an allele of Tcb1-m that is recognized by the Tcb1-female gene leading to directed inhibition of tcb[1] but not the unrecognized Tcb1-m pollen; or (2) Tcb1-m confers a function to pollen not present in tcb[1] allowing it to overcome the barrier set up indiscriminately in Tcb1-f pistils. This second case is preferred since pollen heterozygous for Tcb1-s and tcb[1] haplotypes fertilizes Tcb1-s females, indicating that the Tcb1-female:Tcb1-male match overcomes the Tcb1-female:tcb1-male mismatch[11,12]

Summary

Introduction

A central problem in speciation is the origin and mechanisms of reproductive barriers that block gene flow between sympatric populations. There are two alternative explanations of rejection of tcb[1] pollen: (1) tcb[1] pollen expresses an allele of Tcb1-m that is recognized by the Tcb1-female gene leading to directed inhibition of tcb[1] but not the unrecognized Tcb1-m pollen; or (2) Tcb1-m confers a function to pollen not present in tcb[1] allowing it to overcome the barrier set up indiscriminately in Tcb1-f pistils This second case is preferred since pollen heterozygous for Tcb1-s and tcb[1] haplotypes (using a trisomic line carrying a duplication of the short arm of Chromosome 4) fertilizes Tcb1-s females, indicating that the Tcb1-female:Tcb1-male match overcomes the Tcb1-female:tcb1-male mismatch (the situation is similar for the Ga1 and Ga2 systems)[11,12]

Methods

Results

Conclusion