Disposal of auxiliary cell haploid nuclei during post-fertilization development in Guiryella repens gen. et sp. nov. (Ceramiaceae, Rhodophyta)

Abstract

The marine red alga (Rhodophyta) Guiryella repens gen. et sp. nov. (Ceramiaceae, Ceramiales) is described from the subtidal of Western Australia. Plants are filamentous, with prostrate and upright axes. Branching of the upright axes is typically spiral, with a rotation of approximately 60° between successive branches. Mature axes appear alternate-distichous. Female fertile axes terminate indeterminate branches and are composed of three modified cells, with the subterminal cell bearing two pericentral cells as well as the supporting cell of the four-celled carpogonial branch. The supporting cell also bears a sterile cell. Following fertilization, two auxiliary cells are cut off, one from the supporting cell and one from a pericentral cell. Each of these auxiliary cells receives a diploid nucleus from the carpogonium via small connecting cells. After acceptance of the diploid nucleus, each of the auxiliary cells cuts off a ‘disposal cell’ containing one or two apparently haploid nuclei. These cells rapidly degenerate, but their hyaline remains are obvious in all but the most mature carposporophytes. The remainder of the auxiliary cell divides to form carposporangia, with only a small sterile cell subtending the single gonimolobe. The sterile cells of the procarp (the sterile cell on the supporting cell, the sterile pericentral cell and the apical cell) each divide once to produce two-celled branches. Their development is usually arrested at this stage, but additional cells may be cut off. Spermatangia are produced in compact heads on lateral branch cells. Tetrasporangia, with tetrahedrally-arranged spores, are produced in similar positions to the spermatangia. Two-celled propagules, which presumably repeat the parent phase, are also produced on the tetrasporophyte. Guiryella is clearly related to the propagule-producing genera of the Ceramiaceae, but differs with regard to details of its post-fertilization development. Most notable is the production of a ‘disposal’ cell (although this feature may prove to be more widespread), but the genus is also unique in the arrested development of the inner involucral branches and the production of two lobes of synchronously maturing carpospores without an extensive fusion cell. The method of deleting haploid nuclei from the auxiliary cell via a terminal ‘disposal’ cell, which then degenerates, has not been described previously, but it is suspected to occur in several other genera. The process is analogous to the division (as occurs in several tribes of the Ceramiaceae) of the auxiliary cell into an intercalary ‘foot’ cell (containing haploid nuclei) and the gonimoblast initial. The ‘foot’ cell, however, is typically intercalary, and does not subsequently degenerate.