Megasporogenesis, Megagametogenesis and Ontogeny of the Aril in Cytisus striatus and C. multiflorus (Leguminosae: Papilionoideae)

Abstract

There are few embryological reports on wild legumes and even fewer on their seminal appendages. There are no existing studies on the complete ontogeny of these appendages in Cytiseae, a very important Papilionoideae tribe in Mediterranean ecosystems. In this work megasporogenesis, megagametogenesis and aril ontogeny were studied in Cytisus multiflorus and C. striatus, endemics from the western Mediterranean region. Ovaries and ovules from flower buds, flowers at anthesis and hand cross-pollinated flowers were sectioned with a rotary microtome and studied under light and fluorescence microscopy. A monosporic Polygonum-type of megagametogenesis is observed in both species but with megasporogenesis characterized by formation of a triad of cells after incomplete meiosis. The original cell wall of the megaspore mother cell and triad, including the transverse walls between the latter, are surrounded by a callose layer that isolates them from the surrounding diploid tissue; this callose layer gradually disappears during embryo sac formation. There are no antipodals in the mature embryo sac. Aril ontogeny starts in pre-anthesis with the formation of the aril primordium, and its normal development will occur only after fertilization, more specifically after endosperm initiation. After fertilization, a reactivation of meristem capacity takes place in the aril cells resulting in slow and sparse growth. Later, this type of development gradually decreases but the aril cells continue to grow by cell expansion, which in the last period of seed development is the only type of growth of the aril. In the mature seed, the seminal appendage acquires an irregular U-shape in transverse section, showing vacuolated cells with a large central vacuole that stores lipids and some proteins. Meiotic triad formation is due to a failure in meiosis II of the chalazal cell of the dyad. In Cytisus seeds the aril has a funicular origin with predominantly post-fertilization development, but a normal growth of the endosperm is needed for proper aril development.