Breeding systems, hybridization and continuing evolution in Avon Gorge Sorbus

Area of Science:

• Plant evolutionary biology
• Reproductive biology
• Genetics

Background:

• The genus Sorbus (Rosaceae) exhibits significant genetic diversification driven by interspecific hybridization and polyploidy.
• The Avon Gorge is a recognized global center for Sorbus diversity, featuring both diploid sexual and polyploid apomictic species.
• Understanding the interplay between mating systems, hybridization, and polyploidy is crucial for explaining Sorbus biological diversity.

Purpose of the Study:

• To investigate the complex interactions between mating system variation, hybridization, and polyploidy in generating Sorbus diversity.
• To analyze the mating systems of diploid, triploid, and tetraploid Sorbus taxa within the Avon Gorge.
• To elucidate the evolutionary mechanisms contributing to the unique Sorbus populations in this region.

Main Methods:

• Controlled pollinations were conducted to assess pollen tube growth and seed set in different Sorbus ploidy levels.
• Parent-offspring genotyping was performed on progeny from open and manual pollinations.
• Mating systems of diploid, triploid, and tetraploid Sorbus taxa were analyzed.

Main Results:

• Diploid Sorbus species are characterized by outcrossing and self-incompatibility (SI).
• Triploid taxa function as pseudogamous apomicts, producing genetically uniform offspring, but require pollen from other Sorbus taxa for seed set due to SI, creating opportunities for hybridization.
• Tetraploid taxa are pseudogamous but self-compatible, lacking the obligate intertaxon pollination requirement seen in triploids.

Conclusions:

• Complex mating inter-relationships among Avon Gorge Sorbus taxa are the primary drivers of hybridization and ongoing genetic diversification.
• Self-incompatibility in triploid pseudogamous apomicts mandates interspecific cross-pollination, fostering continued evolution through rare sexual hybridization events.
• The study presents the first evidence of naturally occurring pseudogamous apomictic SI plant populations, suggesting interspecific pollination and relaxed endosperm balance requirements facilitate their persistence, making Avon Gorge Sorbus a model system for SI apomict study.