A plant/fungus and an animal that form a single species?

Question

Two vastly dissimilar organisms turn out to be the same species upon gene sequencing. In this hypothetical animal species one or more components of the life cycle is a fungus, algae, plant, lichen or other vegetative organism. The species may transition from animal to vegetative or vice versa between instars, it might alternate between vegetative and animal generations, or one of the sexes is vegetative.

How feasible would this be to evolve? What selection pressures would promote this adaptation? The ancestral organism doesn't need to be an animal.

(Inspired by Descolada from Speaker for the Dead and Orks from Warhammer 40,000.)

EDIT: here are some more detailed examples.

• In Speaker for the Dead, the capra is a ruminant that grazes exclusively on the capim grass. These are the female and male of the same species, respectively. When the capra graze they are fertilized by the capim pollen.
• Another species in Speaker for the Dead, the suckflies, hatch from the tassels of river reeds. These are the male and female of the same species, respectively.
• The most extreme example in Speaker for the Dead are the pequeninos, in which juvenile males molt into trees to reach maturity.
• In Warhammer 40,000 the orks produce spores which grow into fungi. These fungi produce underground uteri in which new orks gestate.

Answer 1

I would suggest the main pressure would be from a massive shift in sunlight. So when there is lots of sun the creature is able to 'plant' itself, produce chlorophyll, turn very green, lay down some roots and soaks up some rays. This would also allow it time to grow food stores in the form of potato-like starchy tumours to reabsorb later, dramatically changing its appearance into a green lumpy plant. Lying down would remove the need for a solid structure (like a strawberry plant, not a tree) as there wouldn't be much growth vertically. They could also sleep a lot to make maximum use of the abundance of energy, instead of wasting any being awake.

Should then the sun disappear for a long period, because of an eliptical planetary orbit, or a slow spinning planet, it would be an evolutionary advantage to be able to pluck ones feet from the ground, metabilise all the chlorophyll, turning very pale, and be able to move around in the hunt for alternative food sources like an animal. Movement is likely to be achieved through internal hydraulics, moving fluid around the body to push the limbs in the right place. This would make them slow, but strong.

The alternative food sources would also have to be able to survive without sunlight, so I imagine it would eat a lot of mushrooms, and maybe a lot of the less evolved plants, before they starve to death in the dark. It would reabsorb its tumours and likely become very thin. It wouldn't be able to maintain any bark, at least not all over, as it couldn't move, but it would be able to regrow any limb, as long as enough of the rest of the creature survives to provide the energy to do so.

These 2 states would give it the appearance of being 2 separate creatures, depending on the time of year you visited them, leading to the initial confusion.

Reprodution could be asexual through self harming and planting the removed "cutting" or sexually through the usual pollen and seeds method.

I would suggest the descendants were likely plants but that gained the ability to move, maybe first their branches to scoop up what was around them. The extra food that being able to move in the long night provides was advantageous over starving to death standing still so they continued to evolve that way.

Answer 2

Your question reminded me of an organism that does something similar. Slime molds exist most of the time as single cell animals, absorbing food wherever it is, then sporulating. When the nutrients it needs become depleted in an area, the single celled animals clump together and crawl away, looking for new pastures. More information here.