Establishment of plastid-based quadripolarity in spore mother cells of the moss Funaria hygrometrica

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Abstract
Development of a tetrad of meiospores is one of the most widespread examples of geometrically precise cell morphogenesis in plants. We have studied the process in the moss Funaria hygrometrica. Changes leading to a quadripolar organization of the prophase spore mother cell (SMC) start in the archesporial cells several cell generations before meiosis. The number of plastids per cell is reduced to two and these play an increasing part in subsequent mitoses and meiosis. During meiotic prophase, the plastids elongate until they enclose the peripheral nucleus. The nucleus is then drawn back into the centre of the cell as the plastids rotate and ultimately assume a mutually perpendicular configuration. The tips of the plastids thus lie at the vertices of a tetrahedron arranged around the nucleus, which itself becomes deformed into a tetrahedral shape. Quadripolarity has now been set up in anticipation of the two meiotic divisions. The first division spindle is also somewhat tetrahedral, with broad poles oriented perpendicular to one another along two opposite edges of the tetrahedron. As a consequence, the daughter nuclei are, from their inception, mutually perpendicular and elongated along the first spindle poles, ready for the second division, which places one haploid nucleus opposite each of the four plastid tips. Simultaneous cytokinesis then bisects the plastids and generates a tetrad of spores. The morphological evidence thus indicates that the plastids are involved in the development of internal quadripolarity in the outwardly apolar SMCs.