Endopolyploidy Changes with Age-Related Polyethism in the Honey Bee, Apis mellifera

Honey bees (Apis mellifera) exhibit age polyethism, whereby female workers assume increasingly complex colony tasks as they age. While changes in DNA methylation accompany age polyethism, other DNA modifications accompanying age polyethism are less known. Changes in endopolyploidy (DNA amplification in the absence of cell division) with increased larval age are typical in many insect cells and are essential in adults for creating larger cells, more copies of essential loci, or greater storage capacity in secretory cells. However, changes in endopolyploidy with increased adult worker age and polyethism are unstudied. In this study, we examined endopolyploidy in honey bee workers ranging in age from newly emerged up to 55 days old. We found a nonsignificant increase in ploidy levels with age (P < 0.1) in the most highly endopolyploid secretory cells, the Malpighian tubules. All other cell types decreased ploidy levels with age. Endopolyploidy decreased the least amount (nonsignificant) in neural (brain) cells and the stinger (P < 0.1). There was a significant reduction of endopolyploidy with age in leg (P < 0.05) and thoracic (P < 0.001) muscles. Ploidy in thoracic muscle dropped from an average of 0.5 rounds of replication in newly emerged workers to essentially no rounds of replication (0.125) in the oldest workers. Ploidy reduction in flight muscle cells is likely due to the production of G1 (2C) nuclei by amitotic division in the multinucleate striated flight muscles that are essential to foragers, the oldest workers. We suggest that ploidy is constrained by the shape, size and makeup of the multinucleate striated muscle cells. Furthermore, the presence of multiple 2C nuclei might be optimal for cell function, while higher ploidy levels might be a dead-end strategy of some aging adult tissues, likely used to increase cell size and storage capacity in secretory cells.The open access fee for this work was funded through the Texas A&M University Open Access to Knowledge (OAK) Fund

Least square estimates of mean ploidy levels post-emergence (intercept), and change of ploidy over time (slope) for five different worker honey bee (<i>Apis mellifera</i>) tissues: Brain, thoracic (flight) muscle, leg muscle, Malpighian tubules, and stinger.

<p>Adult honey bee workers were collected at different days post emergence (See <i><a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0122208#sec002" target="_blank">Methods</a></i>for details).</p

Least squares estimates of endopolyploid levels in newly emerged honey bee workers (i.e., "Intercept") and rate of change of endopolyploidy per day (i.e., "Slope") for the five tissues studied.

<p>* <i>P</i> < 0.05</p><p>*** <i>P</i> < 0.001</p><p>Least squares estimates of endopolyploid levels in newly emerged honey bee workers (i.e., "Intercept") and rate of change of endopolyploidy per day (i.e., "Slope") for the five tissues studied.</p

Histograms showing fluorescence peaks centered at mean fluorescence channels 25, 50, 100, 200, and 400 produced by 2N, 4N, 8N, 16N and 32N nuclei isolated from flight muscle of (A) a 3-day-old and (B) a 27-day-old worker bee.

<p>The count of nuclei under each peak and the average ploidy level are given immediately below each histogram. The reported significant reduction of ploidy with age (<i>P</i> < 0.01) is reflected in the reduced 8N peak and 16N peak in the older bee.</p