Cytological aspects of haemoglobin and chlorocruorin synthesis in polychaete Annelids

Globins in the marine annelid Platynereis dumerilii shed new light on hemoglobin evolution in bilaterians

BACKGROUND

How vascular systems and their respiratory pigments evolved is still debated. While many animals present a vascular system, hemoglobin exists as a blood pigment only in a few groups (vertebrates, annelids, a few arthropod and mollusk species). Hemoglobins are formed of globin sub-units, belonging to multigene families, in various multimeric assemblages. It was so far unclear whether hemoglobin families from different bilaterian groups had a common origin.

RESULTS

To unravel globin evolution in bilaterians, we studied the marine annelid Platynereis dumerilii, a species with a slow evolving genome. Platynereis exhibits a closed vascular system filled with extracellular hemoglobin. Platynereis genome and transcriptomes reveal a family of 19 globins, nine of which are predicted to be extracellular. Extracellular globins are produced by specialized cells lining the vessels of the segmental appendages of the worm, serving as gills, and thus likely participate in the assembly of a previously characterized annelid-specific giant hemoglobin. Extracellular globin mRNAs are absent in smaller juveniles, accumulate considerably in growing and more active worms and peak in swarming adults, as the need for O2 culminates. Next, we conducted a metazoan-wide phylogenetic analysis of globins using data from complete genomes. We establish that five globin genes (stem globins) were present in the last common ancestor of bilaterians. Based on these results, we propose a new nomenclature of globins, with five clades. All five ancestral stem-globin clades are retained in some spiralians, while some clades disappeared early in deuterostome and ecdysozoan evolution. All known bilaterian blood globin families are grouped in a single clade (clade I) together with intracellular globins of bilaterians devoid of red blood.

CONCLUSIONS

We uncover a complex "pre-blood" evolution of globins, with an early gene radiation in ancestral bilaterians. Circulating hemoglobins in various bilaterian groups evolved convergently, presumably in correlation with animal size and activity. However, all hemoglobins derive from a clade I globin, or cytoglobin, probably involved in intracellular O2 transit and regulation. The annelid Platynereis is remarkable in having a large family of extracellular blood globins, while retaining all clades of ancestral bilaterian globins.

The anatomy of the blood vascular system of the giant vestimentiferan tubeworm Riftia pachyptila (Siboglinidae, Annelida)

The giant dimensions of vestimentiferan Riftia pachyptila (Jones, ) are achieved thanks to the well-developed vascular system. In the vestimentum, there is a complicated net of lacunae, including the brain blood supply and the ventral lacuna underlying the ciliary field. The trunk region has an extensive network of blood vessels feeding the gonads («rete mirabile»). The thick muscular lining of the mesenterial vessels in the trunk and the dorsal vessel in the opisthosome serves as an additional pump, pushing blood into numerous vessels in the segments. It was hypothesized that the blood envelope of the ventral blood vessel in the trunk provides the blood supply to the trophosome. The 3D reconstruction has revealed that there are two vascular systems of the tentacular crown of R. pachyptila. Blood runs into the tentacles via axial afferent vessels, as described earlier only for Riftia, and also via basal ones, as described for other vestimentiferans except Riftia. The basal ones are poorly developed, and the number of lamellar blood vessels is small, indicating a lack of demand for these within huge R. pachyptila. It appears that the presence of these vessels is the preserved ancestral state of Vestimentifera. In different portions of the dorsal vessel, the morphology of the intravasal body varies, depending on function.

Direct evidence that extracellular giant hemoglobin is produced in chloragogen tissues in a beard worm, Oligobrachia mashikoi (Frenulata, Siboglinidae, Annelida)

In Oligobrachia mashikoi, a mouthless and gutless polychaete known as a beard worm, sites of production of extra-cellular giant hemoglobin were examined with whole-mount in-situ hybridization and semi-quantitative RT-PCR. An RNA probe was prepared from mRNA of the A2-globin subunit. Clear signals were obtained from a peritoneal membrane covering the trophosome in the posterior body in all seven individuals examined in this study. In addition, weak signals were observed in the peritoneal membrane covering tissues in the middle part of the body in some individuals. Furthermore, in one individual, signals were obtained in complicated bodies invaginated into the dorsal vessel from a peritoneal membrane that also released signals. The results of RT-PCR regarding the expression levels of four kinds of globin-subunit genes suggest that the main site of hemoglobin production is the peritoneal membrane in the posterior body.

Blood Cells and Blood Cell Development in the Animal Kingdom

Recent findings strongly suggest that the molecular pathways involved in the development and function of blood cells are highly conserved among vertebrates and various invertebrate phyla. This has led to a renewed interest regarding homologies between blood cell types and their developmental origin among different animals. One way to address these areas of inquiry is to shed more light on the biology of blood cells in extant invertebrate taxa that have branched off the bilaterian tree in between insects and vertebrates. This review attempts, in a broadly comparative manner, to update the existing literature that deals with early blood cell development. I begin by providing a brief survey of the different types of blood cell lineages among metazoa. There is now good reason to believe that, in vertebrates and invertebrates alike, blood cell lineages diverge from a common type of progenitor cell, the hemocytoblast. I give a synopsis of the origin and determination of the hematocytoblast, beginning with a look at the hematopoietic organs that house hemocytoblasts in adult animals, followed by a more detailed overview of the embryonic development of the hematopoietic organ. Finally, I compare the process of blood lineage diversification in vertebrates and Drosophila.

The role of the heart-body and of the extravasal tissue in disposal of foreign cells in two polychaete annelids

Sheep red cells injected into the coelom of the polychaetes Neoamphitrite figulus and Arenicola marina are phagocytosed by coelomocytes and conveyed to the heart-body and extravasal ('chloragogen') tissue respectively. In the former case the phagocytes creep into the vessels, penetrate the heart-body tissue and bequeath the sheep red cells to the heart body cells. In Arenicola the phagocytes accumulate on the tips of the blind ending vessels and the red cells are similarly transferred to the extravasal tissues. These observations confirm that the heart-body and extravasal cells function not only in production of haemoglobin but in sequestering foreign material.