In a study published in BMC Biology, researchers from the Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences and Lanzhou University revealed a sophisticated natural selection process that protects closely related spruce trees from the genetic chaos of interbreeding. These findings provide new evidence for taxonomic delimitation, helping scientists distinguish species that appear nearly identical yet remain genetically distinct.

The researchers examined on three varieties of spruce (Picea likiangensis vars. likiangensis, linzhiensis, and rubescens) that naturally hybridize in overlapping ranges on the Tibetan Plateau. By sequencing whole genomes from 193 trees across 23 populations in two distinct transition zones, they investigated how natural selection curb the flow of harmful genes between varieties.

The researchers introduced a powerful new analytical tool called the relative hybrid index (ωH), which measures the ratio of non-synonymous mutations (those that alter protein function) to synonymous mutations (which are functionally neutral).

Across all populations, the ωH values consistently fell below 1.0, ranging from 0.26 to 0.91. This indicates that non-synonymous mutations are systematically purged from hybrid genomes, while neutral changes are allowed to persist.

"Our analysis showed that genes carrying non-synonymous mutations introgress at significantly lower rates than those with synonymous mutations," said SUN Yongshuai of XTBG. "This pattern can only arise if natural selection is actively removing maladaptive genetic variants from hybrid populations."

Strikingly, this filtering effect intensifies near the core range of the recipient variety, Picea likiangensis var. rubescens. In populations closest to pure rubescens stands, ωH dropped to as low as 0.26, revealing exceptionally strong purifying selection against introduced mutations that could disrupt local adaptation.

The researchers also uncovered a significant correlation with the recombination rate. Genomic regions with suppressed recombination exhibited higher levels of donor ancestry and more restricted introgression, suggesting that the physical structure of the genome influences how effectively selection can weed out unwanted variants.

"Our work underscores the power of comparative population genomics to unravel how natural selection shapes introgression and sustains genetic boundaries, even among species that still exchange genes," added SUN.

This study clarifies the evolutionary dynamics of high-altitude spruces and provides a practical toolkit for identifying taxa in other cryptic species complexes.