Litheness of reproductive traits drives perplexing sexual polymorphism and adaptive strategies in Thymus linearis Benth. across diverse habitats

Abstract

Plasticity in floral traits is often associated with sexual polymorphism that reflects adaptations of plant species to different ecological conditions. Here, we studied the adaptive significance of sexual polymorphism on the reproductive ecology and reproductive success of Thymus linearis −a gynodiocious and duo-dichogamous plant species from the Kashmir Himalayas. All eight studied sites exhibited the coexistence of both female and hermaphrodite flowers with significantly different reproductive attributes. Female flowers were relatively smaller (0.27 ± 0.01 mm in diameter) compared to hermaphrodite flowers (0.32 ± 0.02 mm). In sexual accomplishment of T. linearis, we identified intervention of four floral morphs (A, B, C, and D) with unique form and functionality. These floral morphs varied in the number of pollens per flower, pollen viability, pollen variation, pollen load index, and stigma receptivity. The maximum pollen number was recorded in morph A (3373.3 ± 21.95) and the minimum in morph C (95.12 ± 0.01). The maximum pollen viability was recorded in morph D (87.66 ± 0.61 %), and the minimum in morph B and C (0 %). The maximum pollen variation was observed in morph D (60.53 ± 1.35) and the minimum in morph A (4.53 ± 0.165). The pollen load index was maximum in morph B (0.75) and minimum in morph A (0.25). The maximum duration of stigma receptivity was recorded in morph B (7–8 days) and the minimum in morphs A and D (2 days). The aforementioned species exhibited mixed mating favouring xenogamy over autogamy. The seed set recorded in xenogamy was 78.23 ± 1.34 and 54.57 ± 0.17 % for both female and hermaphrodite flowers, while in autogamy, the seed set recorded was 0 and 44.57 ± 0.17 % for both the female and hermaphrodite flowers. The species is mainly pollinated by honey bees and bumble bees. The research findings on the reproductive biology and sexual polymorphism of T. linearis will provide valuable insights for the development of effective conservation and breeding strategies.