Pattern formation in the absence of polarity in Dictyostelium discoideum

Abstract

MANY theories have been advanced as to how the spore–stalk pattern of the slime mould Dictyostelium discoideum is established. Without exception these envisage the polarity of the grex stage of the life cycle as a key factor in specifying the pattern. One group of theories^2–4 sees the combined influence of polarity and the grex tip as signalling positional information^5,6 which determines that the front cells differentiate into prestalk cells and the back cells into prespore cells. An alternative view is that pattern formation occurs by sorting out within the grex of cell types which are pre-determined at an earlier stage of the life cycle^7–11. Grex polarity is also a key factor in one suggested mechanism for sorting-out in that prestalk cells are seen as moving faster than prespore cells and therefore coming to the front of the migrating grex^8,12. Because all these thories clearly suggest that pattern formation ought not to occur in circumstances in which the cell mass is unpolarised, we asked the question: ‘Is polarity necessary for differentiation and pattern formation in D. discoideum?’ We have found that a prestalk–prespore pattern very similar to that seen in the normal life cycle forms in spherical aggregates which, before pattern formation, have no detectable morphological polarity. We also describe the formation of unusual cyst-like structures from patterned spherical aggregates.