The Rim101p/PacC Pathway and Alkaline pH Regulate Pattern Formation in Yeast Colonies

Abstract

Multicellular organisms utilize cell-to-cell signals to build patterns of cell types within embryos, but the ability of fungi to form organized communities has been largely unexplored. Here we report that colonies of the yeast Saccharomyces cerevisiae formed sharply divided layers of sporulating and nonsporulating cells. Sporulation initiated in the colony's interior, and this region expanded upward as the colony matured. Two key activators of sporulation, IME1 and IME2, were initially transcribed in overlapping regions of the colony, and this overlap corresponded to the initial sporulation region. The development of colony sporulation patterns depended on cell-to-cell signals, as demonstrated by chimeric colonies, which contain a mixture of two strains. One such signal is alkaline pH, mediated through the Rim101p/PacC pathway. Meiotic-arrest mutants that increased alkali production stimulated expression of an early meiotic gene in neighboring cells, whereas a mutant that decreased alkali production (cit1D) decreased this expression. Addition of alkali to colonies accelerated the expansion of the interior region of sporulation, whereas inactivation of the Rim101p pathway inhibited this expansion. Thus, the Rim101 pathway mediates colony patterning by responding to cell-to-cell pH signals. Cell-to-cell signals coupled with nutrient gradients may allow efficient spore formation and spore dispersal in natural environments.

Cell Signals, Cell Contacts, and the Organization of Yeast Communities

“…Colonies were viewed from above, below, and the side (after the colony was sliced down the middle), allowing a reconstruction of expression throughout the colony. In an alternative approach, colonies were embedded in plastic or frozen and then sectioned (47,48). Embedded sections allowed the pattern of cell types (ovoid cells, pseudohyphae, and spores) within colonies to be determined (46,48).…”

“…In an alternative approach, colonies were embedded in plastic or frozen and then sectioned (47,48). Embedded sections allowed the pattern of cell types (ovoid cells, pseudohyphae, and spores) within colonies to be determined (46,48). Frozen sections (cryosections) allowed the pattern of expression of lacZ fusion genes within colonies to be visualized.…”

“…Patterns form in diploid colonies that first grow and then sporulate (48). Sporulated cells (asci) are easily distinguishable from nonsporulated cells, so the distribution of sporulating cells in these colonies is obvious from examining embedded sections (Fig.…”

Cryosectioning Yeast Communities for Examining Fluorescence Patterns

“…The methanol fixing step makes the yeast colony rigid 8 and preserves the integrity of the colony; however, it also causes shrinkage 8 that should be taken into account in interpreting the results. We typically see around 30% shrinkage in height of yeast colonies, compared to colonies directly embedded in OCT without fixing 9 .…”

“…As a result, some of the cells at the edges of the section float away, and the sections are more likely to suffer disturbances in the sectioning process ( Figure 4). Although not focused on here, bright-field images of cross-sections of yeast communities may also give us useful information about the state of cells within the community 9 . Whether or not the property of interest is affected by the fixing step has to be examined on a case-by-case basis.…”