Comparative flower development of Juglans regia, Cyclocarya paliurus and Engelhardia spicata: Homology of floral envelopes in Juglandaceae

Abstract

Homology among reproductive structures is pivotal for understanding angiosperm evolution. In Juglandaceae, homologies of floral parts have been disputed due to morphological variability in flowers. Previous interpretations were based mainly on mature flowers and lacked developmental studies. We investigated the unisexual flower development of Juglans regia, Cyclocarya paliurus and Engelhardia spicata using scanning electron microscopy. The 'floral envelope' of staminate flowers in J. regia and C. paliurus consists of a bract and several tepals. Six tepals are initiated in a whorled pattern in J. regia, whereas four to six tepals are arranged in a variable pattern in C. paliurus. The three-lobed bract of E. spicata results from the cleavage of an entire bract, rather than adnation of the bract and its bracteoles. Pistillate flowers of J. regia and C. paliurus, subtended by a bract and a horseshoe-shaped bracteole, usually have four tepals initiating simultaneously in one whorl. The organogenesis patterns of the 'floral envelope' and stamens in staminate flowers show greater diversity, indicating different degrees of reduction among genera. Our floral developmental data support the division of Juglandaceae into two subfamilies and then further into three tribes.