How do algae and fungi shift to sexual reproduction just before onset of adverse conditions

I would have to check up on the veracity of your premise. However, I could conjecture an answer if this is indeed true.

Nature appears to be guided through homeostasis. Evolution is a balancing act. It has nothing to do with directional progress. It has to do with balance. The earth creates species as required to promote homeostasis. Niches open and are filled. The analogy is a river filling a lake until it reaches its boundaries (the confines of natural selection) then it overflows to fill another lake. These lakes represent niches.

For nature to remain in some sort of balance, it must have reservoirs from which to tap from. As we know, most of the DNA encoding species remains a mystery. The amount of DNA that actually encodes for proteins is a small portion of the total DNA. In fact, there is major homology of DNA between species in this “junk DNA”. So what is it’s homeostatic purpose?

It is possible that this “reserve” DNA serves a Grand purpose. What we consider as predictive adaptation (as your question suggests) is sensitive to minor fluctuations in a homeostatic environment. Man is too simple to conceptually understand these because they happen in real time at the cellular level. Our understanding is at a very simplistic macroscopic level. We have no idea what each cell of our bodies are doing right now.

Therefore, natural triggers will always remain outside our superficial consciousness. In fact, it is the cellular activity that dictates that consciousness. We know so very little. My supposition is that Nature is in balance beyond our knowledge. We can never understand what creates our knowledge since that would be like a finger pointing at itself.

In my opinion.