Analyses of Hybrid Viability across a Hybrid Zone between Two Alnus Species Using Microsatellites and cpDNA Markers.

Jan Šmíd,Bohumil Mandák,Jan Douda,Karol Krak

doi:10.3390/genes11070770

Abstract

Diploid Alnus glutinosa s. str. and autotetraploid A. rohlenae form a narrow hybrid zone in a study area in southern Serbia, which results in triploid hybrid formation. The vast majority of previous studies have been focused on studies of maternal plants, but the offspring resulting from their crossing have not been much studied. Here, we use the variability of microsatellites and chloroplast DNA between these species and their putative hybrids to create an overall picture of the development of the hybrid zone and its predicted type. To elucidate the gene transfer within both species, the origins of individual ploidies and especially the role of triploid hybrids, a germination experiment was carried out linked with a flow cytometry study of the resulting seedlings. The tension zone model seems to offer the most adequate explanation of our observations, with selection against triploid hybrids and the spatial positioning of the hybrid zone. Despite selection against them, the triploid hybrids play an important role in the exchange of genes between the two species and therefore serve as a bridge for introgression. The presence of fertile triploids is essential for enriching the haplotype diversity between these species and for the development of new genetic lineages.

Highlights

It is generally accepted that most hybrid zones are maintained in a balance between naturally occurring selection and the diffusion of genes by dispersal across the boundary species [1,2]
While in the Iberian Peninsula, the tetraploid cytotype was described as Alnus lusitanica Vít, Douda and Mandák, in the western Balkan Peninsula, we find another tetraploid species, Alnus rohlenae Vít, Douda and Mandák
Our study yielded a number of important outcomes concerning the establishment and maintenance of a hybrid zone lying at the margin of the distribution range of tetraploid A. rohlenae

Summary

Introduction

It is generally accepted that most hybrid zones are maintained in a balance between naturally occurring selection and the diffusion of genes by dispersal across the boundary species [1,2]. Three main types of hybrid zone are recognised. These are (1) a bounded hybrid superiority zone, (2) a tension zone and (3) a mosaic hybrid zone. The type of hybrid zone formed depends mainly on hybrid fitness, selection against them and gene flow between the parents and the hybrids [1,3]. In a bounded hybrid superiority zone, the hybrids have greater fitness and they occupy the habitat intermediate between the habitats of the two parents. The mechanisms that maintain species barriers in many hybrid zones can be much more complicated than in these three abovementioned examples [4,5,6,7,8]. Some hybrid lines can have lower fitness due to genetic complications, but hybrid fitness is usually inconstant and highly variable, and such populations often contain some relatively fit and fertile genotypes with the potential to contribute new genetic variation to future

Results

Discussion

Conclusion