Embryonic Development of Parthenogenetic and Sexual Eggs in Lower Termites

SIMPLE SUMMARY: It is important to compare the developmental stages of lower termites and recognize the differences between parthenogenetic and sexual eggs in order to describe their embryonic development concisely. In contrast with sexual eggs, parthenogenetic eggs have a lower rate of development and hatching success. Moreover, several genes differentially expressed between the two types of embryos have been identified. By addressing such a problem in the current research, the lack of understanding in the differences and the developmental processes between the two categories of eggs can be resolved. The aim of the study was to evaluate the development, discover the differences, and determine the gene expression patterns related to the variances between the two types of eggs. The authors discovered a number of genes with differential expression between the two categories of embryos, which explain these differences. These results present important insights into the reproductive processes of termites and highlight the need to contribute further research in this area to achieve a better understanding of termite biology. ABSTRACT: Worldwide, termites are one of few social insects. In this research, the stages of embryonic development in the parthenogenetic and sexual eggs of Reticulitermes aculabialis and R. flaviceps were observed and described. In R. flaviceps, the egg development of the FF and FM groups happened during the early phases of development, whereas in R. aculabialis, this appeared mainly during the late phase of development. The variance in the number of micropyles between the R. flaviceps FF colony type and the R. aculabialis FF colony type was statistically significant. Five stages of egg development were found in both types of R. aculabialis but only the sexual eggs of R. flaviceps. In R. flaviceps, 86% of the parthenogenetic eggs stopped growing during the blastoderm development, with the yolk cell assembling frequently in the center of the egg. According to the results of the single-cell transcriptome sequencing, we investigated the egg-to-larval expression level of genes (pka, map2k1, mapk1/3, hgk, mkp, and pax6) and indicated that the levels of essential gene expression in RaFF were considerably higher than in RfFF (p < 0.05). We also discovered that the oocyte cleavage rate in the FF colony type was considerably lower in R. flaviceps compared to R. aculabialis, which gave rise to a smaller number of mature oocytes in R. flaviceps. During ovulation in both species, oocytes underwent activation and one or two cleavage events, but the development of unfertilized eggs ceased in R. flaviceps. It was shown that termite oocyte and embryonic development were heavily influenced by genes with significant expressions. Results from the databases KEGG, COG, and GO unigenes revealed the control of numerous biological processes. This study is the first to complete a database of parthenogenetic and sexual eggs of R. flaviceps and R. aculabialis.