Nuclear and cell fusion cause polyploidy in the megagametophyte of common cypress, Cupressus sempervirens L.

Abstract

In common cypress, Cupressus sempervirens L., the megagametophyte persists in mature seeds as a polyploid endosperm containing cells with even and odd series of DNA contents: 1C, 2C, 3C, 4C, 5C etc., where C is the amount of DNA in the haploid genome. In this study, cytometrical, histological and cytochemical investigations were performed in order to determine the behavior of megagametophyte nuclei during the reproductive cycle. Unexpected nuclear alterations due to a continuous process of nuclear fusion were observed in the megagametophyte, leading to polyploidization and consequently to intense food-reserve synthesis. During the free nuclear stage, the megagametophyte exhibited only sporadic nuclear fusion and limited food-reserve production. When cellularization took place, multinucleated compartments were observed in which nuclei fused, producing odd and even series of DNA contents as proved by flow-cytometric analysis. This polyploidization process considerably increased after fertilization and during embryo development, and was accompanied by increased food-reserve synthesis. During these later stages, fusion mainly involved nuclei of contiguous cells and was preceded by the disintegration of their adjacent walls. Mitoses with incomplete phragmoplast differentiation were also observed to yield polyploid nuclei. Finally, in mature seeds the endosperm still exhibited multinucleate cells and fusion nuclei, and contained high amounts of storage products. The results are interpreted as an alteration of DNA contents in the megagametophyte cells in relation to specific metabolic activity during seed development.