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It has been known for some time that fungi are more closely related to animals than to plants (ref, 1997). The split from plants occurred one billion years ago and 35 million years later the fungi and animals parted company…or did they? Candida endosymbionts can be found in a variety of beetles and even humans!
Mouse genome: 3,400 million nucleotides
Human genome: 3,300 million nucleotides
Neurospora crassa (pink bread mold), 38.6 million nucleotides, 88 full genomes could fit into a mouse nucleus.
Saccharomyces cerevisiae (bakers yeast), 12 million nucleotides, 283 full genomes could fit inside a mouse nucleus.
Eukaryotic endosymbiont of Guillardia theta, 551,264 nucleotides, ~ 6168 full genomes could fit inside a mouse nucleus.
Genes within the endosymbionts that are redundant to the host or those affecting endosymbiotic autonomy would be deleted out over time, reducing the endosymbiotic genomes still further.
Can symbionts themselves contain symbionts? They can and they do. A classic example is the termite which contains eukaryotic protists within their gut that helps them digest wood . These protists, in turn, contain bacteria that do the actual work of digesting the cellulose. Spirochaetes can attach to the cell surface of the protists as ectosymbionts (ref). Visit my 1999 website and review my models on how spirochaetes acting as ectosymbionts may have led to the development of both mitosis and meiosis (figs. 30, 31, and 37).