SEXUAL TETRAMORPHISM IN THYMELAEA HIRSUTA (THYMELAEACEAE): EVIDENCE OF THE PATHWAY FROM HETERODICHOGAMY TO DIOECY AT THE INFRASPECIFIC LEVEL

After more than 5 years of study on natural populations of Thymelaea hirsuta (L.) Endl. (Thymelaeaceae), four distinct sexual phenotypes are shown to occur: protogynous, protandrous, subandroecious (male), and subgynoecious (female) individuals. The four populations studied differ significantly in the relative abundance of sexual types. Plant size and sexual phenotype are independent in three populations. Each sexual form produces viable seeds. Growth and morphogenesis of the aerial vegetative and sexual parts are described and found not to be related to sexual phenotype. The system, called sexual tetramorphism, combines characteristics of subdioecy (subandroecious and subgynoecious individuals) and heterodichogamy (protogynous and protandrous individuals). High fruit production in females and protandrous plants vs. low fruit production in males and protogynous plants provides evidence of a strong tendency toward functional dimorphism. The probable specialization of protogynous types toward the male function and the obvious specialization of protandrous types toward the female function suggest an evolution from heterodichogamy to dioecy, sexual tetramorphism being the intermediate stage. Tetramorphism in T. hirsuta provides further support for the hypothesis of an evolutionary pathway from heterodichogamy to dioecy.