Embryology of Laurales: a review and perspectives

Embryological studies of Laurales were reviewed to clarify whether embryology provides any evidence for a circumscription of Laurales as well as for the relationships within the order that have been suggested by molecular evidence, where and how embryological characters have evolved, and which embryological data are lacking or in need of further study. Comparisons with related groups and in particular with Magnoliales indicate that Laurales comprising seven families (Atherospermataceae, Calycanthaceae, Gomortegaceae, Hernandiaceae, Lauraceae, Monimiaceae s. str. and Siparunaceae) are probably defined by a multi-celled ovule archesporium and a crushed mesotesta. Within Laurales, 16 embryological characters are found to be diversified, including the number of microsporangia (four or two), mode of anther dehiscence (by slits or valves), the number of archesporial cells in an ovule (one, two or more), and so on. Their respective evolutions were traced on a phylogenetic tree generated from molecular and morphological data, supporting that Calycanthaceae represent the earliest branch and are sister to a clade comprising six remaining families with a valvate anther dehiscence as already suggested. However, because of a lack of sufficient data from each family of Laurales, embryological characters often have alternative possibilities with respect to where they have evolved. More extensive and/or intensive studies of individual families are needed to clarify their exact evolutionary relationships.