EMBRYOLOGY AND SYSTEMATIC POSITION OF HETEROPYXIS (MYRTALES)

We present here the first study of the embryology of Heteropyxis, in order to evaluate its relationships with Myrtaceae, a family in which it has often been placed. On the basis of embryology, Heteropyxis is undoubtedly very closely allied to Myrtaceae, but is distinct in possessing 1) a bisporic Allium type embryo sac, 2) a condition in which the two microsporangia of the theca are remotely separate from each other and dehisce by their respective longitudinal slits instead of by a common slit, and 3) an ovule and seed that has wings on both ends. All of these embryological characteristics are apomorphic features; they contrast with many other characteristics of the genus, such as its completely superior ovary and separate staminal traces, which are clearly primitive ones. Like the unique monotypic Psiloxylon, an endemic of Mauritius, Heteropyxis clearly diverged early from the evolutionary line leading to Myrtaceae before the origin of the common ancestor of that family (excluding Heteropyxis and Psiloxylon). Heteropyxis could logically be regarded either as a subfamily of Myrtaceae distinct from all other members of the family or as a separate but related family. We shall return to this question when our review of the embryological features of Psiloxylon has been completed.