Chromosomal changes more important than niche differentiation in a Pulmonaria hybrid

Hybridization has been a key driver of speciation in plants, often in combination with the duplication of chromosomes sets (allopolyploidy). Changes in chromosome number can provide intrinsic reproductive barriers, in contrast to extrinsic barriers such as niche differentiation or distance-based isolation. Though studies suggest that allopolyploids may not need extrinsic reproductive barriers for speciation, they appear to promote speciation greatly in homoploid hybrids.

In an article published in Annals of Botany, lead author Sandra Grünig and colleagues used both plastid and nuclear sequences to characterize the genetic variation of Pulmonaria helvetica (2n=24), a poorly understood putative hybrid native to low elevation Swiss forests, in order to understand its origins and population structure. The researchers also used comparative ecological modelling to investigate whether niche differentiation played an important role in the plant’s reproductive isolation.

Genotyping of P. helvetica showed a well-delineated group with respect to its closest relatives. Its gene pool is consistent with a hybrid origin, with P. mollis (2n=18, 22) as the maternal ancestor and P. officinalis (2n=16) as the paternal one. P. helvetica’s identity as an independent species was also supported by limited introgression in contact zones with other Pulmonaria species, suggesting reproductive isolation, though the chromosomal rearrangements involved remain elusive.

Pulmonaria helvetica’s current range was completely covered by ice until circa 16,000 years ago, so hybridization is presumed to have taken place after the recolonization of forests on the Swiss plateau sometime around 13,000 years ago, when secondary contact was made between populations of the parental species. Comparative ecological modelling failed to show significant differentiation between the niches of the hybrid and its parent species, at least for abiotic factors.

Surprisingly, the hybrid has become abundant within its narrow range. “The effective expansion of such a recent species may have been promoted by hybridization that is known to mask the load of deleterious recessive alleles fixed in parental populations and yield heterosis,” write the authors. Overall, the results paint a picture of a hybrid for which niche differentiation has had a limited impact, but chromosomal changes reducing gene flow may have been key.