Comparative ultrastructure of the spermatogenesis of three species of Poecilosclerida (Porifera, Demospongiae)

Archaeocytes in sponges: simple cells of complicated fate

Archaeocytes are considered a key cell type in sponges (Porifera). They are believed to be multifunctional cells performing various functions, from nutrient digestion to acting as adult stem cells (ASCs). Thus, archaeocytes are mentioned in discussions on various aspects of sponge biology. As presumed ASCs of an early-diverged animal taxon, archaeocytes are of great fundamental interest for further progress in understanding tissue functioning in metazoans. However, the term 'archaeocyte' is rather ambiguous in its usage and understanding, and debates surrounding archaeocytes have persisted for over a century, reflecting the ongoing complexity of understanding their nature. This article presents a comprehensive revision of the archaeocyte concept, including both its historical development and biological features (i.e. taxonomic distribution, characteristics, and functions). The term 'archaeocyte' and its central aspects were introduced as early as the end of the 19th century based on data mainly from demosponges. Remarkably, despite the general lack of comparative and non-histological data, these early studies already regarded archaeocytes as the ASCs of sponges. These early views were readily inherited by subsequent studies, often without proper verification, shaping views on many aspects of sponge biology for more than a century. Taking into account all available data, we propose considering the archaeocytes as a cell type specific to the class Demospongiae. Clear homologues of archaeocytes are absent in other sponge classes. In demosponges, the term 'archaeocytes' refers to mesohyl cells that have an amoeboid shape, nucleolated nuclei, and non-specific inclusions in the cytoplasm. The absence of specific traits makes the archaeocytes a loosely defined and probably heterogeneous cell population, rendering the exhaustive characterisation of the 'true' archaeocyte population impossible. At the same time, the molecular characterisation of archaeocytes is only beginning to develop. Stemness and almost unlimited potency have always been at the core of the traditional archaeocyte concept. However, currently, the most consistent data on archaeocyte stem cell function come only from developing gemmules of freshwater sponges. For tissues of adult demosponges, the data favour a two-component stem cell system, in which archaeocytes may cooperate with another stem cell population, choanocytes. Simultaneously, cells with archaeocyte morphology function as macrophages in demosponges, participating in the food digestion cycle and immune defence. Such cells should be denoted with the more neutral term 'nucleolar amoebocytes', as the term 'archaeocyte' not only describes the morphology of a cell but also introduces the proposition of its stem nature. Thus, the future usage of the term 'archaeocyte' should be limited to cases where a cell is shown or at least presumed to be a stem cell.

Global patterns in symbiont selection and transmission strategies in sponges

Sponges host dense and diverse communities of microbes (known as the microbiome) beneficial for the host nutrition and defense. Symbionts in turn receive shelter and metabolites from the sponge host, making their relationship beneficial for both partners. Given that sponge-microbes associations are fundamental for the survival of both, especially the sponge, such relationship is maintained through their life and even passed on to the future generations. In many organisms, the microbiome has profound effects on the development of the host, but the influence of the microbiome on the reproductive and developmental pathways of the sponges are less understood. In sponges, microbes are passed on to oocytes, sperm, embryos, and larvae (known as vertical transmission), using a variety of methods that include direct uptake from the mesohyl through phagocytosis by oocytes to indirect transmission to the oocyte by nurse cells. Such microbes can remain in the reproductive elements untouched, for transfer to offspring, or can be digested to make the yolky nutrient reserves of oocytes and larvae. When and how those decisions are made are fundamentally unanswered questions in sponge reproduction. Here we review the diversity of vertical transmission modes existent in the entire phylum Porifera through detailed imaging using electron microscopy, available metabarcoding data from reproductive elements, and macroevolutionary patterns associated to phylogenetic constraints. Additionally, we examine the fidelity of this vertical transmission and possible reasons for the observed variability in some developmental stages. Our current understanding in marine sponges, however, is that the adult microbial community is established by a combination of both vertical and horizontal (acquisition from the surrounding environment in each new generation) transmission processes, although the extent in which each mode shapes the adult microbiome still remains to be determined. We also assessed the fundamental role of filtration, the cellular structures for acquiring external microbes, and the role of the host immune system, that ultimately shapes the stable communities of prokaryotes observed in adult sponges.

Ultrastructural research of spermiogenesis in two sponges, Crellomima imparidens and Hymedesmia irregularis (Demospongiae): New evidence of sperms with acrosome in sponges

Details of spermatogenesis and sperm organization are often useful for reconstructing the phylogeny of closely related taxa of invertebrates. Here, the spermiogenesis and the ultrastructure of sperm were studied in two marine demosponges, Crellomima imparidens and Hymedesmia irregularis (order Poecilosclerida). In C. imparidens and H. irregularis, we found bundles of microtubules arranged along the nucleus during spermiogenesis. These bundles derived from the basal body of axoneme, reaching the apical pole of the cell. In C. imparidens, the microtubules surround the nucleus, forming the manchette. In H. irregularis, the microtubules pass along only one side of the cell periphery. During spermiogenesis, the nucleus stretches and elongates. In both species, the nucleus is twisted into a spiral structure. We suppose that the manchette of microtubules could be responsible for controlling the elongation and shaping of the sperm nucleus to a helical form and for the twisting and/or condensation of chromatin in these sponges. The spermatozoon of both species has an elongated shape. Its apical part has an acrosome, which is dome-shaped in C. imparidens and flattened and lenticular in H. irregularis. The cytoplasm of the spermatozoa contains some small mitochondria, and proximal and distal centrioles arranged at an angle to each other. There is a small volume of residual cytoplasm with dark glycogen-like granules. The axoneme of the spermatid and the flagellum of the sperm of both sponges is located in the deep tunnel-like cytoplasmic depression. The comparison of spermatozoa morphology of different species of the order Poecilosclerida demonstrates that the knowledge of variation within genera and families can give valuable insights into the significance of many characters proposed for phylogenetic studies of this order.