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

Special Issue: Genetic Basis of Phenotypic Variation in Drosophila and Other Insects

Next-generation sequencing provides a nearly complete genomic sequence for model and non-model species alike; however, this wealth of sequence data includes no road map [...

ANCOVA results for genome size range compared to λ p-values across taxa numbers.

<p>ANCOVA results for genome size range compared to λ p-values across taxa numbers.</p

Genome size estimates for 87 species of Drosophilidae.

<p>Genome size estimates for 87 species of Drosophilidae.</p

The mode and tempo of genome size evolution in the subgenus <i>Sophophora</i>

<div><p>Genome size varies widely across organisms, with no apparent tie to organismal complexity. While genome size is inherited, there is no established evolutionary model for this trait. Hypotheses have been postulated for the observed variation in genome sizes across species, most notably the effective population size hypothesis, the mutational equilibrium hypothesis, and the adaptive hypothesis. While much data has been collected on genome size, the above hypotheses have largely ignored impacts from phylogenetic relationships. In order to test these competing hypotheses, genome sizes of 87 <i>Sophophora</i> species were analyzed in a comparative phylogenetic approach using Pagel’s parameters of evolution, Blomberg’s K, Abouheif’s C_mean and Moran’s I. In addition to testing the mode and rate of genome size evolution in <i>Sophophora</i> species, the effect of number of taxa on detection of phylogenetic signal was analyzed for each of these comparative phylogenetic methods. <i>Sophophora</i> genome size was found to be dependent on the phylogeny, indicating that evolutionary time was important for predicting the variation among species. Genome size was found to evolve gradually on branches of the tree, with a rapid burst of change early in the phylogeny. These results suggest that <i>Sophophora</i> genome size has experienced gradual changes, which support the largely theoretical mutational equilibrium hypothesis. While some methods (Abouheif’s C_mean and Moran’s I) were found to be affected by increasing taxa numbers, more commonly used methods (λ and Blomberg’s K) were found to have increasing reliability with increasing taxa number, with significantly more support with fifteen or more taxa. Our results suggest that these comparative phylogenetic methods, with adequate taxon sampling, can be a powerful way to uncover the enigma that is genome size variation through incorporation of phylogenetic relationships.</p></div

Comparative phylogenetic output for <i>Sophophora</i> phylogeny.

<p>Comparative phylogenetic output for <i>Sophophora</i> phylogeny.</p

Boxplots for each phylogenetic analysis.

<p>Raw values from comparative phylogenetic tests are plotted for each group of taxa. There is no clear pattern with increasing taxa number for Pagel’s parameters of evolution or Blomberg’s K; however, there is an increase in values for both Moran’s I and Abouheif’s C_mean. These differences are tested statistically in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0173505#pone.0173505.s002" target="_blank">S2 Table</a>.</p

Flying High—Muscle-Specific Underreplication in Drosophila

Drosophila underreplicate the DNA of thoracic nuclei, stalling during S phase at a point that is proportional to the total genome size in each species. In polytene tissues, such as the Drosophila salivary glands, all of the nuclei initiate multiple rounds of DNA synthesis and underreplicate. Yet, only half of the nuclei isolated from the thorax stall; the other half do not initiate S phase. Our question was, why half? To address this question, we use flow cytometry to compare underreplication phenotypes between thoracic tissues. When individual thoracic tissues are dissected and the proportion of stalled DNA synthesis is scored in each tissue type, we find that underreplication occurs in the indirect flight muscle, with the majority of underreplicated nuclei in the dorsal longitudinal muscles (DLM). Half of the DNA in the DLM nuclei stall at S phase between the unreplicated G0 and fully replicated G1. The dorsal ventral flight muscle provides the other source of underreplication, and yet, there, the replication stall point is earlier (less DNA replicated), and the endocycle is initiated. The differences in underreplication and ploidy in the indirect flight muscles provide a new tool to study heterochromatin, underreplication and endocycle control

Boxplots of significance values for λ and Blomberg’s K analyses.

<p>Plotted significance values from phylogenetic signal tests of λ and Blomberg’s K decrease as the number of taxa in the analyses increase, most notably above 15 taxa. These are tested for significance in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0173505#pone.0173505.s003" target="_blank">S3 Table</a>.</p

Phylogenetic generalized least squares results for genome size and climatic variables.

<p>Phylogenetic generalized least squares results for genome size and climatic variables.</p