STRUCTURAL MODIFICATION OF SPINDLE POLE BODIES DURING MEIOSIS-II IS ESSENTIAL FOR THE NORMAL FORMATION OF ASCOSPORES IN SCHIZOSACCHAROMYCES-POMBE - ULTRASTRUCTURAL ANALYSIS OF SPO MUTANTS

In order to characterize the morphological steps defined by sporulatio n (spo) genes during the formation of ascospores in the fission yeast Schizosaccharomyces pombe, we performed an electron microscopic study of the ultrastructure of the spindle pole body (SPB) and of the develo pment of the forespore membrane during the second meiotic division (me iosis II) in sporulation-deficient (spo) mutants (spo4, spo5, spo14 an d spo18). No difference was found in terms of the function and the str ucture of the SPB during the first meiotic division (meiosis I) betwee n the four mutants and wild-type cells. However, during meiosis II, th e spo4 and spo18 mutants underwent nuclear division but in neither cas e were the SPBs modified nor were forespore membranes formed. The SPBs of the spo18 mutant diminished in size after meiosis II and eventuall y disappeared after 18 h in sporulation medium. By contrast, the SPBs of the spo4 mutant remained unchanged even after an 18-h incubation. T he outer plaques of SPBs of spo5 and spo14 mutants were sufficiently m odified to allow them to initiate development of the forespore membran e, but the membrane had an abnormally expanded lumen and did not enclo se the nuclei during meiosis II. The spo5 mutant produced anucleate sp ore-like bodies while the spo14 mutant formed unorganized structures w ith irregular peripheries which, presumably, contained spore-wall prec ursors, instead of anucleate spore-like bodies. We conclude that the m odification of the SPB is essential for the formation of ascospores an d at least two genes (spo5 and spo14) participate in the development o f the forespore membrane. The defective phenotypes define discrete ste ps in the development of ascospores, which proceeds via steps defined by the mutant spo4 spo18, spo14 and spo5 genes respectively. Our obser vations provide further substantial evidence that the SPB plays a pivo tal role in the normal development of ascospores in yeasts.