Unidirectional diploid–tetraploid introgression among British birch trees with shifting ranges shown by restriction site‐associated markers

Abstract

Hybridization may lead to introgression of genes among species. Introgression may be bidirectional or unidirectional, depending on factors such as the demography of the hybridizing species, or the nature of reproductive barriers between them. Previous microsatellite studies suggested bidirectional introgression between diploid Betula nana (dwarf birch) and tetraploid B. pubescens (downy birch) and also between B. pubescens and diploid B. pendula (silver birch) in Britain. Here, we analyse introgression among these species using 51 237 variants in restriction site‐associated (RAD) markers in 194 individuals, called with allele dosages in the tetraploids. In contrast to the microsatellite study, we found unidirectional introgression into B. pubescens from both of the diploid species. This pattern fits better with the expected nature of the reproductive barrier between diploids and tetraploids. As in the microsatellite study, introgression into B. pubescens showed clear clines with increasing introgression from B. nana in the north and from B. pendula in the south. Unlike B. pendula alleles, introgression of B. nana alleles was found far from the current area of sympatry or allopatry between B. nana and B. pubescens. This pattern fits a shifting zone of hybridization due to Holocene reduction in the range of B. nana and expansion in the range of B. pubescens.

Genomics of hybridization and its evolutionary consequences

“…Zohren et al . () develop a new method for genotyping tetraploid individuals across a large number of RAD loci and use this to show that unidirectional introgression has occurred from two diploid species into a widespread tetraploid species of birch ( Betula ). This is in keeping with Stebbins’ rule () that when introgression occurs across a ploidy barrier, it normally proceeds from a diploid into a tetraploid, rather than in the reverse direction.…”

Molecular ecology studies of species radiations: current research gaps, opportunities and challenges

“…In fact, a growing number of studies demonstrate that applying WGS to studies of speciation and radiation is entirely feasible, as long as an assembled and annotated reference genome and sufficient resources are available (The Heliconius Genome Consortium 2012; Brawand et al 2014;Ellegren 2014;Lamichhaney et al 2015;Christe et al 2017;McGee et al 2016;Novikova et al 2016). Although not covered specifically in our study, WGS approaches are also certainly feasible in species groups originating from genome doubling, with available case studies spanning from recent autopolyploids to allopolyploids (products of past hybridization events; Zohren et al 2016) to paleopolyploids with ancient traces of whole-genome duplications (e.g. Christe et al 2017).…”

“…There is considerable potential to extend this type of research to polyploids and taxa with variation in ploidy level, provided that appropriate statistical tools are used to detect and analyse genetic loci and allelic variation (e.g. Arnold et al 2016;Zohren et al 2016). Although perhaps of less concern in animals and fungi, variation in ploidy is significant in many plant lineages (Soltis & Soltis 2016).…”

Interspecies gene flow is a key evolutionary process for the adaptation of long-lived species to new environmental conditions

“…between Anguilla anguilla L. and A. rostrata Lesueur (Wielgos et al, 2014), between Populus fremontii S. watts. and P. angustifolia James (Currat et al, 2008 and references therein), or between Betula pubescens Ehrh (a tetraploid invading species) and two diploid resident species, B. nana L. and B. pendula Roth (Zohren et al, 2016). In oaks, similar rates of bidirectional nuclear introgression were obtained on average between Quercus suber L. and Q. ilex L. when the species were analysed over their whole range, but substantial variation was identified according to loci and contact areas (Lumaret & Jabbour-Zahab, 2009).…”