Potential chromosomal introgression barriers revealed by linkage analysis in a hybrid of Pinus massoniana and P. hwangshanensis

Shuxian Li,Ying Chen,Handong Gao,Tongming Yin

doi:10.1186/1471-2229-10-37

Abstract

BackgroundExploring the genetic mechanisms underlying speciation is a hot topic in modern genetics and evolutionary studies. Distortion of marker transmission ratio is frequently ascribed to selection against alleles that cause hybrid incompatibility. The natural introgression between P. massoniana and P. hwangshanensis and their distribution ranges lead to the emergence of the two species as desirable organisms to study the genetic mechanisms for speciation.ResultsUsing seeds sampled from trees at different elevations, we consistently detected sharp decreases in seed germination rates of trees in the hybrid zone, which might be due largely to the hybrid incompatibility. A genetic map was established using 192 megagametophytes from a single tree in the hybrid zone of the two species. Segregation distortion analysis revealed that the percentage of significant-segregation-distortion (SSD) markers was extremely high, accounting for more than 25% of the segregating markers. The extension range, the distortion direction, and the distortion intensity of SSD markers also varied dramatically on different linkage groups.ConclusionsIn this study, we display the potential chromosomal introgression barriers between P. massoniana and P. hwangshanensis. Our study provides a valuable platform for conducting genome-wide association of hybrid incompatible QTLs and/or candidate genes with marker transmission ratio distortion in the hybrid.

Highlights

Exploring the genetic mechanisms underlying speciation is a hot topic in modern genetics and evolutionary studies
We aim to reveal the potential chromosomal introgression barriers between P. massoniana and P. hwangshanensis by building a genetic map using megagametophytes from a single tree sampled in the hybrid zone and to identify regions of the map displaying extreme segregation distortion
Since segregation distortion demonstrates the uneven transmission ratio of alleles on the alternate chromosomes in the mapping parent and is related to hybrid incompatibility [18], once these SSD markers are mapped onto linkage groups, the chromosomal regions acting as potential introgression barriers will be revealed

Summary

Introduction

Exploring the genetic mechanisms underlying speciation is a hot topic in modern genetics and evolutionary studies. The natural introgression between P. massoniana and P. hwangshanensis and their distribution ranges lead to the emergence of the two species as desirable organisms to study the genetic mechanisms for speciation. A biological species is defined as a group of natural populations that mate and produce offspring with one another, but do not breed with other populations. Inter-specific hybridization is a common natural scenario observed both in plants and animals, which roughly occurs in 10% of animal species and 25% of plant species [2]. Inter-specific mating may lead to introgression [3]. Introgression may cause merging of the hybridization species; at the other extreme, introgression may lead to selection for conspecific mating, and enlarge the reproductive isolation [5]. Studies suggested that hybrids acted as introgression filters, allowing beneficial genes to filter

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