The parasitic plant Balanophora has a really weird chloroplast genome

I wonder what the plastids in Balanophora do. Presumably their reduced function, whatever remains of it, has released some purifying selection that restrains GC bias in other taxa. How many of these plastid proteins are actually functional?

Way back in the last 1990s. I was involved in a project to characterize the chlroplast genome of Rafflesia and some of these other really weird holoparasitic plants. I was trying to use PCR to pull out the ribosomal gene from these genomes. As you might imagine that was a tough task not knowing how AT-rich there were. In fact I only got a few pieces of that genome. We got enough to show that they had a ribosomal RNA gene which is what you might expect to remain even if all photosynthetic genes had decayed. But we were stymied to find protein coding genes. It is good many years later to look back and understand why.
Regarding why they have AT bias? Who really knows. Some have postulated its an energy budget thing but that doesn’t make sense for a parasite. What I do know is that all of the weird holoparasites seem to maintain a plastid with some DNA so we know that photosynthesis isn’t the only critical function. They seem to still do starch synthesis in their plastids and this is where they store energy so energy storage could be the primary selective reason they maintain these organells despite having thrown away their photosynthetic function potentially more than 65 million years ago.

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I wonder what the plastids in Balanophora do. Presumably their reduced function, whatever remains of it, has released some purifying selection that restrains GC bias in other taxa. How many of these plastid proteins are actually functional?

According to the paper, there are 19 genes in the plastid genome. While they are highly divergent, they found evidence (based on dN/dS ratios and expression data) of moderate functional constraint for most of the 19 genes.