Albinism and inbreeding depression in seedlings of the tropical tree, Shorea laxa

Abstract

Investigating the level of inbreeding depression is essential to understanding population viability and extinction risk. Albino or chlorophyll-deficient mutants are one of the strongest manifestations of inbreeding depression in plant species, although it is rare to encounter them in a tropical forest. However, during January 2014, we observed a high frequency of albino seedlings in a natural population of Shorea laxa in Lambir Hills National Park in Sarawak, Malaysia. We investigated the fate and compared the cross types of these albino seedlings versus green-leaf seedlings by combining a field survey and genetic analysis using microsatellites. Overall, the albino seedlings survived at least a few months after germination, although they exhibited a 10-fold higher mortality rate than green-leaf seedlings within 19 weeks after germination. Microsatellite analysis confirmed that most albino seedlings (98.3%) were derived from self-pollination. The overall estimated selfing rate was high (61.3%), indicating that reproductive isolation occurred in the mother tree. The observed segregation rate of green:albino types was consistent with the complementation models, indicating that a combination of two dominant genes at two loci are responsible for the green-leaf phenotype in S. laxa. We also found that one albino seedling was derived from outcrossing, suggesting that at least one deleterious gene is maintained in the population. The inbreeding depression of selfed seedlings was exposed as lower initial height of seedlings, which resulted in lower seedling survival. These findings indicate that the population of S. laxa carries recessive deleterious genes and suffers inbreeding depression in selfed offspring.