The Bothrosome and Endoplasmic Network
Background
One feature unique to the zoospore to vegetative cell transition in thraustochytrids (and other labyrinthulomycetes) is the production of the ectoplasmic network (EN) – branched extensions of the plasma membrane connected to the cell body by a unique organelle known as a bothrosome or sagenogenetosome – which is involved in the search for and attachment to food sources by vegetative cells (Hamamoto & Honda, 2019; Iwata & Honda, 2018). The EN seems functionally analogous to the rhizoids of chytrid fungi (perhaps the ancestral state of fungal hyphae, (Laundon et al., 2020)) or to animal filopodia. However, the EN likely represents an independent instance of the evolution of surface attachment and the identity, evolution, and functions of proteins in the EN and bothrosome remain to be revealed. Ultrastructural characterization of the EN by transmission electron microscopy reveals a wall-less, membrane-bound compartment that may contain ‘net elements’ described as vesicles or internal membrane cisternae, but no cytoplasmic organelles like ribosomes or mitochondria (Perkins, 1973). Actin may function in the delivery of cargo between the cell body and the distal regions of the EN (Fossier Marchan et al., 2018).
We combined time course proteomics and microscopy to characterize the transition from zoospore to a settled vegetative cell with an EN in Aurantiochytrium limacinum ATCC MYA-1381
![](https://you.stonybrook.edu/labyrinthulomycetes/files/2023/10/Picture1.png)
![](https://you.stonybrook.edu/labyrinthulomycetes/files/2023/10/Picture2.png)