A comparative study of seed morphology, dormancy, and germination of four closely-related Aristolochia subgenus Siphisia species (Aristolochiaceae, Piperales): A test of two hypotheses on ecological changes in species within a lineage through geologica

This research examined two opposing views on changes in physiological and ecological requirements and tolerances of species within the same clade that evolved in response to changes in climate during and following breakup of the “Arcto-Tertiary” forest. The hypothesis of the late Daniel I. Axelrod is that physiological and ecological characteristics of modern taxa have not changed from those of their Tertiary ancestors. The hypothesis of Jack A. Wolfe, on the other hand, is that taxa have diverged in their physiological and ecological characteristics as they were exposed to climatic changes through geological time. The two hypotheses were evaluated via a comparative study of seed dormancy and germination characteristics of four closely-related Aristolochia species in subgenus Siphisia that diverged from a common ancestor in the Tertiary: the California endemic A. californica, the Appalachian endemic A. macrophylla, the southeastern U.S. species A. tomentosa, and the east Asian A. manshuriensis, sister species to A. macrophylla . The embryo in all species is underdeveloped and physiologically dormant; thus, the seeds have morphophysiological dormancy (MPD). However, they differ in their dormancy-breaking and germination requirements. Seeds of A. californica require low temperatures (e.g., 10°C) to come out of dormancy and for embryo growth and seed germination. In contrast, seeds of A. macrophylla, A. tomentosa, and A. manshuriensis require cold stratification (e.g., 5°C) to come out of dormancy, but their embryos require temperatures of 15/6°C or higher for growth and subsequent seed germination. Thus, seeds of the temperate rainy-climate species A. macrophylla, A. manshuriensis, and A. tomentosa have nondeep simple MPD, and those of the Mediterranean-climate species A. californica, have deep complex MPD. These results support Wolfe's view on physiological and ecological changes in taxa within a lineage.