Re: Could Indian pipes have dropped photosynthesis through evolution?

I'm not aware that there is fossil evidence for the evolutionary history of 
Indian pipe (Monotropa uniflora) but an explanation can be reasoned out. First 
of all, Indian pipe is not a saprophyte, which obtains energy from dead 
organic matter. Indian pipes were once assumed to be saprophytes but we now 
know that they are parasites. Indian pipe relies on a mycorrhizal fungus which 
is also connected to a tree. Indian pipe depends on both the fungus and tree. 
The fungus transports photosynthates (food) and mineral nutrients from the 
tree to the Indian pipe. Indian pipe is termed an epiparasite or a 
mycoheterotrophic plant.

The term saprophyte is obsolete now that fungi are not considered plants. It 
would be virtually impossible for a plant to digest dead organic matter, which 
is predominantly cellulose. Plants have cell walls made of cellulose so they 
would also digest themselves. Fungi have cell walls made of chitin so they 
have no problem digesting cellulose.  

It seems clear that at one time in the past, an ancestor of Indian pipe was 
photosynthetic and had a symbiotic relationship with a mycorrhizal fungus. The 
majority of plant species have such symbiotic relationships. The mycorrhizal 
fungus also had a symbiotic relationship with tree roots. You might be right 
that the Indian pipe ancestor may have been unable to do enough photosynthesis 
on its own because it grew in deep shade. Therefore, it may have depended 
mainly on the photosynthates it obtained from the tree. At some point, the 
Indian pipe lost its ability to photosynthesize due to a mutation. If a gene
(s) is not required for survival for a species, then it could be lost by 
accident during evolution. Nonphotosynthetic (albino) mutants occur rarely in 
photosynthetic plants, they just don't survive in nature because 
photosynthesis is essential. Albino corn seedlings can be kept alive by 
feeding them sugar. 

There is research on the genome of nonphotosynthetic parasitic plants that 
show that they still have some, but not all, genes needed for photosynthesis. 
Thus, it is clear they were once photosynthetic and lost that ability during 
evolution. 

If you studied bacteria and evolution, you may have also learned about the 
endosymbiont theory. It states that chloroplasts were originally 
photosynthetic bacteria that were engulfed by a nonphotosynthetic cell and 
became incorporated as part of the cell.



References


Re: Are there any plants that do not produce/release oxygen?


Re: How does Indian Pipe influence its community ?


Re: What is a good plant to test the effects of sugar or sweetener?


Molecular Evolution Research on Mycoheterotrophic Plants


Extreme specificity in epiparasitic Monotropoideae (Ericaceae): widespread 
phylogenetic and geographical structure


Endosymbiont Theory