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Abstract
The tapetum in anthers is a tissue that undergoes programmed cell death (PCD) during the production of pollen. We observed two types of autophagy prior to cell death. In Lobivia rauschii (Cactaceae), tapetum cells showed plant-type autophagosomes-autolysosomes, which have been found previously exclusively in root meristem cells. The autophagic structures were formed by a network of tubules which apparently merged laterally, thereby sequestering a portion of the cytoplasm. The organelles observed in the sequestered material included multilamellar bodies, which have not been reported earlier in these organelles. By contrast, Tillandsia albida (Bromeliaceae) tapetum cells contained no such organelles but showed plastids that might possibly carry out autophagy, as they contained portions of the cytoplasm similar to the phenomenon reported earlier in Phaseolus and Dendrobium. However, the ultrastructure of the T. albida plastids was different from that in the previous reports. It is concluded that in L. rauschii classical plant macroautophagy was involved in degradation of the cytoplasm, while in T. albida such classical macroautophagy was not observed. Instead, the data in T. albida suggested the hypothesis that plastids are able to carry out degradation of the cytoplasm.
Highlights
I thoroughly discuss ciliary transition zone (TZ) evolution, highlighting many overlooked evolutionarily significant ultrastructural details
It stemmed from realising that discoverers of Rhodelphis (Gawryluk et al 2019), a remarkable heterotrophic flagellate with an unseen relict plastid unexpectedly related to red algae, had overlooked key aspects of its TZ structure that give compelling evidence for an evolutionary relationship with both glaucophyte algae and another heterotroph of uncertain affinity, Picomonas (Seenivasan et al 2013)
Eukaryote algae with plastids located instead within the rough endoplasmic reticulum (ER) and with an intervening periplastid membrane (PPM) were placed instead in a new kingdom Chromista thought to have evolved by one secondary enslavement of a plant cell whose plasma membrane became the periplastid membrane; algae with plastids in the cytosol but with three bounding membranes were placed in kingdom Protozoa, where euglenoid algae remain, though dinoflagellate algae are in Chromista as their ancestor secondarily lost the PPM (Cavalier-Smith 2018)
Summary
I thoroughly discuss ciliary transition zone (TZ) evolution, highlighting many overlooked evolutionarily significant ultrastructural details. In so doing I show that elements of the TZ hub-lattice and distal nonagonal fibre structures discovered in Rhizaria (Cavalier-Smith et al 2008a, 2008b, 2009) exist in Plantae, some other corticates, and a majority of eukaryote lineages and are of broader significance for eukaryote TZs than hitherto appreciated; correct some past interpretative errors of TZ and centriole comparative anatomy; adduce evidence for numerous overlooked ultrastructural homologies within and across phyla; argue that a filamentous skeleton of the dense TP is conserved to some degree across all eukaryotes; and provide a novel explanation of the ancestral role of the acorn-V filament complex.
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