## 3 Discussion  
As described in the introduction, morphological patterns in microbial colonies arise due to processes at different scales (Figure 1). In this paper, we developed a 2D ABM and used it to quantify the biophysical impact of budding cell division on the overall shape, size and spatial organization of growing yeast colonies. The novelty of our approach is that we explicitly model the mechanical interactions that arise due to budding cell division. An additional novelty lies in the metrics we developed to quantify spatial organization of cells within the colony. Moreover, results from these metrics reveal the impact of budding division and nutrient limitation on patterns of displacement and spatial rearrangement of cells leading to emergent local and global morphological properties of growing yeast colonies. In Section 2.1, our findings reveal that budding division does not impact global, large-scale structures of growing yeast colonies (Figure 3B,C), or the birth location of cells within the colony (Figure 4 that grew more slowly (average doubling time of \(\sim\)123 min compared to \(\sim\)105 min for nutrient-rich colonies) (Figure 11A-D). In addition, cells in nutrient-limited colonies were much more likely to be born near the edge of the colony, where nutrients are more readily available, than near the middle, where nutrients are highly depleted (Figure 11F). Moreover, since lack of nutrients slows down cell cycle progression, most actively dividing cells are near the boundary of the colony. As such, our model captures emergent patterns of colony organization due to changes in local cell-cell interaction dynamics caused by rapid expansion and spatial rearrangement of newly born cells at the colony boundary. An interesting extension of our model would be to compare simulation results with our simple model of nutrient limitation to simulation results representing a nutrient field that changes in time due to diffusion and consumption by live cells. Our results also reveal that together, budding and nutrient-limited growth facilitate subclony connectivity. We observe that nutrient-limited growth in our simulations results in better defined and more contiguous subcolonies than in the nutrient-rich case (Figure 11A-D). This is likely due to the effects of nutrient limitation on the cell cycle for densely packed regions of the colony. Furthermore, the nutrient limitation condition creates high variance in the final number of cells that will be in each subcolony at a particular time (Figure 11E). We believe this variation is caused by different patterns of movement of the founder cell. Namely, if the founder cell is moved to areas of high nutrient availability due to interactions with neighboring cells, it can produce more offspring. Otherwise, if cell-cell interactions cause it to remain stationary in low-nutrient regions, its cell cycle progression will be slowed resulting in fewer total offspring. These observations further support the idea that nutrient-limited growth together with budding division result in colonies with the most well-defined subcolony shapes. We hypothesize that these contiguous subcolonies are precisely the discernible sectors observed in experimental yeast colonies. As such, our mathematical framework will provide valuable insight for generating hypotheses on sectoring behavior that can be compared to experimental studies. This variation in subcolony size from nutrient limitation will have a different impact for cells grown on a plate (i.e., physical media) compared to those in liquid culture (where cells are in a well-mixed liquid environment) and our work identifies population structure as a novel and unappreciated difference in these two common experimental conditions. As such, changes in population structure are a new lens through which experiments in these conditions can be compared. In addition to its impact on local spatial organization and subcolony structure, resource-driven structuring in microbial groups has been linked to colony fitness and survival [8; 29].