Paper: Shi et al. (2011). Diet and cell size affect queen-worker differentiation through DNA methylation in honeybees (Apis mellifera, Apidae). PLoS One.
What are some other “types” of honeybees, other than queens and workers?
Drones are male bees that result from a haploid unfertilized egg. They do not have stingers, nor do they gather nectar or pollen. Intercaste is another type of bee that has an intermediate phenotype between that of a queen or worker.
What prompted the authors to investigate the effects of DNA methylation on honey bee development?
Recent studies had suggested that DNA methylation was a factor involved in the caste differentiation of honeybees. Honey bees had been found to have DNA methyltransferases that are orthologs of the vertebrate Dnmt1 and Dnmt3 enzymes, and silencing of the Dnmt3 gene was found to result in a higher proportion of queens. The authors were interested in examining whether royal jelly exerts its action through changes in methylation, mediated by Dnmt3. The authors were also interested in examining whether different cell sizes could also contribute to the caste differentiation process, as there had not been much attention previously devoted to this factor.
Considering Table 1, what are the main question(s), experiment, results, and possible conclusions? How do conclusions support the article’s title claim?
Question: What effect(s) do(es) food and cell type have on the queen-worker differentiation process?
Experiment: Examined larval development when they were fed royal jelly versus worker jelly. Also examined development when larvae were grown in different sizes of cells.
Results: There were changes in methylation at 4 sites as the duration of royal jelly feeding increased, indicating that royal jelly duration is correlated with methylation at CpG locations.
Conclusion: This experiment demonstrates a correlation between the duration of royal jelly consumption or growth within a queen cell and development into queens, which provides some support for the article’s title claim. However, another experiment must be done in order to demonstrate convincingly that the methylation pattern changes are what cause the larvae to develop into either queens or workers. Until then, the effects of diet and cell size cannot be said to function “through DNA methylation” in order to affect differentiation.
Consider Figure 2. What is/are the experiment, results, conclusions, and possible inferences? How do the conclusions support the title claim?
Experiment: Larvae were fed with royal jelly for different durations (3, 4, or 5 days). The levels of Dnmt3 enzyme activity, Dnmt3 expression, and methylation of the dynactin p62 gene were measured, and the differentiated state of the honeybees were observed at day 6.
Results: A longer duration of royal jelly feeding was found to be correlated with an increase in the percentage of adults that differentiated into queens. Dnmt3 activity and expression, as well as methylation of dynactin p62, were shown to decrease as the duration of royal jelly feeding increased.
Conclusions: While this experiment does not explicitly establish causation between methylation activity and differentiation into queens, the fact that 5 days of royal jelly feeding was sufficient to make all of the larvae differentiate into queens provides strong support that an element of the royal jelly is responsible for the differentiation into a queen phenotype.
Consider Figure 3. What does cell size refer to? What was the experimental question, the experiment, results, and conclusions? Do the conclusions support the title?
Cell size refers to the type of cell in which the larva develops – queen cell or worker cell.
Question: What is the effect of cell size on the methylation patterns of honeybees? Does a change in methylation patterns correlate with a change in the percentage of adults that develop into queens?
Experiment: Reared larvae in either worker or queen cells, and assayed them at 3 and 5 days of age. Measured the levels of Dnmt3 activity, Dnmt3 RNA expression levels, and percent methylation of the dynactin p62 gene.
Results: Larvae that were reared in the queen cells had lower levels of Dnmt3 activity, mRNA expression, and overall methylation in the dynactin p62 gene relative to larvae raised in worker cells. However, no queens resulted from larvae reared in queen cells, with 19% differentiating into intercastes.
Conclusions: Rearing larvae in queen cells is not sufficient to differentiate honeybees into queens. However, queen cell size is correlated with decreased levels of Dnmt3 activity, expression, and methylation. These data may suggest that the cell size may be another component of the differentiation process, as rearing larvae in non-worker cells (i.e. queen cells) resulted in a lower proportion of workers overall.
Propose a model, incorporating the conclusions made from the data presented in the article and the information presented in class, that explains how the difference in diet between queen and worker larvae could result in their phenotypic differences.