Vegetative and reproductive development of MediterraneanGulsonia nodulosa(Ceramiales, Rhodophyta) and its genetic affinities

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Gulsonia nodulosa was originally described as Crouania annulata by Berthold in 1882 and transferred to Gulsonia by Kylin in 1956 because its lateral indeterminate branches originated from the basal cells of whorled branchlets rather than being borne directly on the axis as a fourth branch. This study provided photographic evidence for the characters portrayed by other researchers and added new information concerning carposporophyte development. Each node bore four whorled branchlets that were usually up to five times trichotomously branched, terminated by one to four dichotomous branches and were formed in successive rows offset by 45°. Rhizoidal filaments were produced singly on basal cells of whorled branchlets and overlapped earlier-formed rhizoids while remaining undivided throughout their length. Plants were dioecious. Spermatangial parent cells arose as oppositely or quadraverticillately branched filaments on whorled branchlets, with each forming two to three spermatangia. Mature spermatangia were ovoid with a distal nucleus and a proximal vacuole, and released a single spermatium. Female thalli were procarpic and produced four-celled, L-shaped carpogonial branches proximally on young lateral branches of limited growth. After fertilization, the carpogonium cut off a terminal capping cell and a lateral connecting cell, whereas the supporting cell enlarged and divided transversely to produce a uninucleate auxiliary cell. The connecting cell fused with the auxiliary cell, its nucleus then dividing, with one of the derived nuclei entering the auxiliary cell and the other extruded in a residual cell that remained attached to the carpogonium. The diploidized auxiliary cell divided transversely into a diploid gonimoblast initial and a narrow bilobed foot cell in which the lobe nearest the carpogonium contained a diploid nucleus and the distal lobe received the original haploid nucleus, which usually divided into two nuclei. The first gonimolobe was produced laterally toward the carpogonium, followed by a second gonimolobe from the opposite side. A third gonimolobe sometimes formed midway between the first two. Involucral filaments surrounded the mature gonimolobes. Analyses of DNA sequences for two nuclear-encoded and two plastid-encoded genes indicated that G. nodulosa is more closely related to Ptilocladia and Euptilocladia species than to Crouania, and support hypothesized relationships of Crouanieae genera on the basis of morphological characters.

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