"It is usually assumed that the trypanosomatid plastid shared a common origin with that of euglenids, but Δ4 desaturase phylogenies suggest that it could have originated via an independent, tertiary endosymbiosis involving a haptophyte alga. It is also possible that ancestors of the Trypanosomatidae initially possessed a primary plastid that later was replaced by a secondary or tertiary plastid." – Bodyl et al 2010 J Parasitol (pdf)I could go on for many, many pages about the implausibility of most entirely unnecessary serial plastid symbiosis theories; I could go on for pages yet on how little a single gene phylogeny means these days; I could go over the typical first few lectures on phylogenetic reconstruction and the fundamental principle of parsimony. But instead, I've quickly thrown together a diagram highlighting the KEY problem with Bodyl et al.'s hypothesis:
Taxa in black – non-photosynthetic and non-plastid-bearing.
Or any reason to suspect they might.
*to be fair, they are (I hope) talking about a plastid in their ancestry, but those things are seldom lost completely due to inevitable strong dependencies.
In fact, unlike apicomplexans, trypanosomes are nested firmly within a completely non-photosynthetic phylum in a predominantly non-photosynthetic subgroup of an almost-exclusively non-photosynthetic supergroup. Furthermore, the many possible phylogenies of euglenid evolution overwhelmingly support a single symbiotic event; character evolution supports this further, in one of the few cases where there's actually little room for dispute. Endosymbiosis may not be excessively rare, but it ain't common either, particularly in a full-fledged form involving vast transfer of plastid genes to the nucleus AND mechanisms of plastid targeting of the synthesised proteins. Too many an ambitious biologist completely forget about targeting, or that there's actual cell biology happening around their beloved gene sequences.
For a properly scientific and civil demolition of an earlier iteration of this ridiculous idea, see Leander 2004 Tr Microbiol (pdf). That smell of something burning? No need to worry – probably just coming from the link.
Lastly, as ridiculous as this hypothesis is and as amusing as it is that this actually survived peer review (no offense to J Parasitol, but phylogenies and evolution do not seem to be their strong point based on some other cases...), I fully support it being published. It is the excessive censorship of atypical theories rather than sketchy papers that "stiffles [scientific] thought"...
(Note: I would've submitted this to the high IF Journal of Are You Fucking Kidding, but I'm out of hard liquor and would thereby fail the author instructions...)
References
Bodył, A., Mackiewicz, P., & Milanowski, R. (2010). Did Trypanosomatid Parasites Contain a Eukaryotic Alga–Derived Plastid in Their Evolutionary Past? Journal of Parasitology, 96 (2), 465-475 DOI: 10.1645/GE-1810.1
LEANDER, B. (2004). Did trypanosomatid parasites have photosynthetic ancestors? Trends in Microbiology, 12 (6), 251-258 DOI: 10.1016/j.tim.2004.04.001
So why are they proposing an ancestral plastid in the first place? Have they identified possible plastid-derived genes in the nuclear genome?
ReplyDeleteNot entirely sure... maybe because of the name "kinetoplastid"? (derives from 'kinetoplast', little to do w plastid) But that would be mocking them, more seriously this might be a relic of the Cabozoa Hypothesis, which was definitively murdered in cold blood a couple years ago (before this Bodyl et al paper) – if Euglenids shared a plastid origin with Rhizarian Chlorarachniophytes, then it would phylogenetically make sense for Kinetoplastid ancestors to have had the plastid too. But it's pretty bloody clear that Euglenid and Chlorarachniophyte green plastids have separate secondary endosymbiosis events, so the tryp plastid idea makes even less sense.
ReplyDeleteAs for genes, you can ALWAYS find plastid genes in anything if you look hard enough... LGT is a bitch. It takes more than just a gene or five to justify an organellar endosymbiosis, and some, especially those buried too deep in molecular biology and bioinformatics, forget that.
With algal symbiosis, people often forget basic things like plastid targetting, which can't just happen from a couple transferred genes. This is actually the reason why thus far, the sea slug algal symbiosis is not a true permanent relationship; even if gene transfer did clearly occur, there is no evidence for a plastid targetting system, and thus the plastid cannot be permanent. The press releases (and authors, of course) for one reason or another failed to mention any of that.
So even lateral transfer of plastid-derived genes is not enough to show endosymbiosis; you need plastid-*targetting* genes (and mechanism) for that.
(things I wouldn't've ever thought about had I not wandered into an endosymbiosis lab by accident...)