Organelle Preservation and Protoplast Partitioning in Fossil Angiosperm Leaf Tissues

Abstract

Analyses of the ultrastructural states of preservation seen in fossil angiosperm mesophyll cells indicate that plastids are more commonly found in portions of protoplasts (= microprotoplasts) lacking nuclei than in microprotoplasts containing nuclei. Of the 4,498 cells examined in serial thick sections, less than 1.8% had continuous protoplasts, while the remaining 98.2% had from 2 to 3 microprotoplasts or lacked any discernable internal structure. Where nuclei and plastids were found in the same cell (21.3%), only 29.5% had these two organelles in the same microprotoplast, while 70.5% had nucleate microprotoplasts lacking plastids and enucleate microprotoplasts with well preserved plastids. Over 64% of all the cells sampled lacked nuclei but had well preserved plastids. Neobotanical studies indicate nuclei are more stable than plastids, but during microprotoplast formation plastids are more stable in enucleate protoplasts than in those containing nuclei. Senescence and necrosis are triggered by the nuclear genome. The preferential preservation of plastids in fossil angiosperm tissues is interpreted to be the result of the isolation of part of the plastid population from nuclear control due to microprotoplast formation. This hypothesis also may explain the apparent longevity of nuclei over plastids in other fossil cells (spores, pollen, algal tissues) where protoplasmic partitioning may be less frequent.