What does a mixed population of Pinus sibirica and P. pumila from the southern Baikal region suggest about the structure of their hybrid zone?

Pinus sibirica and P. pumila have a vast hybrid zone, and its structure and maintenance model have not yet been considered. This study was carried out in the southern-western part of the hybrid zone. On the Khamar-Daban Ridge, there is a vigorous belt of mountain taiga forest, where the prevalent species is P. sibirica and where P. pumila produces a subalpine tree belt. The border between these belts is blurred, and a mixed zone of the species is formed, resulting in interspecies hybridization. Field observations, seed efficiency determination, and analysis of mt- and cpDNA markers were used to elucidate the pattern of this hybridization. Pinus sibirica and P. pumila hybridization was determined as bidirectional, and P. pumila was mainly the mother plant. Hybridization transforms to an introgression, and based on the life form, the studied hybrids were divided into two groups: intermediate and pumila-like cup-shaped forms. Both morphological types of hybrids had significantly fewer sound seeds per cone than the parental species. However, pumila-like hybrids had more sound seeds per cone compared to intermediate hybrids. We observed a positive correlation between hybrid seed efficiency and elevation. Summarizing the results of this study and those previously obtained enabled us to show different hybridization patterns in different parts of the hybrid zone, suggesting a mosaic model of hybrid zone maintenance.