Histochemical properties of the spherulocytes of Galleria mellonella L. (Lepidoptera : Pyralidae)

Hemocyte Clusters Defined by scRNA-Seq in Bombyx mori: In Silico Analysis of Predicted Marker Genes and Implications for Potential Functional Roles

Within the hemolymph, insect hemocytes constitute a heterogeneous population of macrophage-like cells that play important roles in innate immunity, homeostasis and development. Classification of hemocytes in different subtypes by size, morphology and biochemical or immunological markers has been difficult and only in Drosophila extensive genetic analysis allowed the construction of a coherent picture of hemocyte differentiation from pro-hemocytes to granulocytes, crystal cells and plasmatocytes. However, the advent of high-throughput single cell technologies, such as single cell RNA sequencing (scRNA-seq), is bound to have a high impact on the study of hemocytes subtypes and their phenotypes in other insects for which a sophisticated genetic toolbox is not available. Instead of averaging gene expression across all cells as occurs in bulk-RNA-seq, scRNA-seq allows high-throughput and specific visualization of the differentiation status of individual cells. With scRNA-seq, interesting cell types can be identified in heterogeneous populations and direct analysis of rare cell types is possible. Next to its ability to profile the transcriptomes of individual cells in tissue samples, scRNA-seq can be used to propose marker genes that are characteristic of different hemocyte subtypes and predict their functions. In this perspective, the identities of the different marker genes that were identified by scRNA-seq analysis to define 13 distinct cell clusters of hemocytes in larvae of the silkworm, Bombyx mori, are discussed in detail. The analysis confirms the broad division of hemocytes in granulocytes, plasmatocytes, oenocytoids and perhaps spherulocytes but also reveals considerable complexity at the molecular level and highly specialized functions. In addition, predicted hemocyte marker genes in Bombyx generally show only limited convergence with the genes that are considered characteristic for hemocyte subtypes in Drosophila.

Correlation of hemocyte counts with different developmental parameters during the last larval instar of the tobacco hornworm, Manduca sexta

We determined the changes in hemocyte titer and in the abundance of hemocyte types of the tobacco hornworm Manduca sexta during the fourth and fifth larval stadium and the beginning of the pupal stadium. As we analyzed the samples of individual insects at daily intervals, we were able to correlate phenotypical features, body weight, as well as total protein content and lysozyme activity in the hemolymph with the observations on hemocytes. In the course of the fifth larval stadium, the hemocyte titer decreased slightly and declined further after pupation. Using calculated values for total hemocyte numbers, females had about five times and males three times more hemocytes in the circulating population at the beginning of the wandering stage (in the middle of the fifth larval stadium) than immediately after the last larval--larval molt (from the fourth to the fifth larval stadium). This sexual difference was mainly due to an increase in the number of plasmatocytes, which was more prominent in females than in males. Granular cells were dominant in early fifth larval stadium while plasmatocytes were the most abundant cells in pupae. Oenocytoids and spherule cells disappeared during the wandering stage. Lysozyme activity in the hemolymph rose to a maximum during the wandering stage, with females having lysozyme values twice as high as those for males. These changes in lysozyme activity, however, did not correlate with the increase of total hemolymph protein titer which occurred already at the beginning of the wandering stage. We postulate that changes in hemocyte titers are under direct hormonal control, which has to be proven in future experiments.