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Ascobolus  -  one more time!

Coprophilous fungi are great subjects for student classes.  They are very easy to collect.  Just incubate recently deposited herbivore dung on moist paper towels in a plastic box and for the next several weeks or longer you will get a startling succession of  remarkable and colourful fungi covering a number of Divisions e.g Zygomycota, Ascomycota, Basidiomycota, Myxomycota.   Ascobolus is one of the most commonly encountered coprophilous fungi.  This tiny Discomycete has fruitbodies (apothecia) measured in mm.  Ascobolus, however,  produces nice big asci with purple ascospores that can easily be seen using the least fancy of the bright field microscopes.

In the photomicrograph above (slightly doctored with Adobe) you can see a cluster of asci of Ascobolus.  In the three mature asci the spores are purple and in two of the young asci the spores are still colourless (hyaline).   In the tiny, youngest ascus (bottom right) the spores have not even begun to form.   The empty shell of an old ascus that has discharged its spores can be seen faintly (centre).

Students: I have asked this question before but no harm to reflect on it again!

Quickie review:   Nuclear fusion and meiosis takes place in the ascus.   Reduction division produces four nuclei.  Each nucelus then divides mitotically to produce eight nuclei and the protoplasm cleaves around these nuclei to produce the ascospores.   By far the largest group of species in the Ascomycota have eight ascospores per ascus.   A  few  species of Ascomycota have four ascospores per ascus or less due to aborted nuceli (e.g Tuber).  In a number of species with additional synchronous mitotic divisions we can end up with 16, 32, 64, 128, 256 etc ascospores.

QUESTION: The easiest solution to form spores in Sac fungi would be to cleave around the meiotic products and end up with four ascospores.   In most cases this doesn't happen and we have one more (mitotic) division.  SO!  Eight ascospores is better than four. The question is WHY?    And if eight is better than four why is 16 etc not better than eight?