OPTICAL AND ELECTRON MICROSCOPE STUDY OF
HETEROSPORIC THALLI (CARPOSPORES/TETRASPORES)
IN Gigartina pistillata (Gmel.) Stackh (Rhodophyta)
L. Pereira, J. F. Mesquita, J. D. Santos Dias
Botanical Department, Laboratory of Electron Microscopy and Phycology, University of Coimbra, 3000 Coimbra - PORTUGAL
Introduction
General morphology of the “heterosporic thallus”
It is known that the majority of red algae (Rhodophyta-Florideophycidae) display a
triphasic life cycle where three generations develop in regular sequence: dioecious
gametophytes, carposporophyte and tetrasporophyte. Generally, the gametophyte and
tetrasporophyte are independent and isomorphic thalli, whereas the carposporophyte rises
from female gametophyte to which it remains attached. However, abnormal correlations
among these generations have been sometimes reported in Ceramiaceae (1) and much more
rarely in other families (2, 3). In the scope of a larger work on Gigartinales of the
portuguese coast, thalli of Gigartina pistillata, exhibiting both tetrasporangial sori and
cystocarps, i.e., tetraspores and carpospores (“heterosporic thalli”), were found. In order to
clarify the nature and function of these reproductive structures, its morphological and
cytological study was carried out.
The peculiar heterosporic thallus clearly shows an “hybrid morphology”, where typical tetrasporic and well
developed cystocarpic “plants” seem to be associated (fig. 1). In the tetrasporangial part of this thallus, flattened or
lens-shaped dark patches stand out from the branches (fig. 1b, arrows), which are in all similar to those we can see
on the standard monosporic tetrasporophytic thalli. The tetrasporangia are plunged in the tissue of the thallus
forming sori, the release of which frequently occurs at the maturation. As to the carposporic part of the thallus, it
shows a very distinctive feature given by abundant and small branches supporting cystocarps in intercalary or
terminal position (fig. 1a, arrows).
Material and Methods
For electron microscopy, small samples of the thalli were fixed in a mixture of
glutaraldehyde (2.5%)/paraformaldehyde (2%) in 0.1M phosphate buffer, pH 6.8, posfixed
with OsO4 (0.1%) in the same buffer and embedded, for a long time, in Spurr’s resin.
Staining by toluidine blue, black Sudan or PAS method and Thiery’s technique were used
for cytochemical approach in light and electron microscopy, respectively.
a
b
Fig. 1. Heterosporic thallus showing both tetrasporic (on the right) and carposporic
(on the left) zones. Enlarged views of cystocarps (1b arrows) and
tetrasporangial sori (1a, arrows) can also be seen.
Ultrastructure and cytochemistry of the reproduction zones of the “heterosporic thallus”
The differentiation of carposporangia and tetrasporangia in these heterosporic thalli, comparatively to both standard plants of G. pistillata and other species of Gigartinaceae4, was studied by optical and electron
microscopy. The most significative results can be synthetised as follows: during carposporogenesis, a multinucleated cell begins to isolate gonimoblast cells which, progressively, evolve into mature carposporangia. In this
differentiation process, apart from current ultrastructural features of Rhodophyceae (chloroplasts, floridean starch grains …), the Golgi apparatus (dictyosomes) (arrows) has shown to be the most intervenient cell organelle,
producing “cored vesicles” (arrowheads) and polysaccharidic fibrillar vacuoles which, definitively, contribute to carposporangial wall formation (fig. 2, 2a). Excepting its meiotic cruciate division to produce tetraspores (fig. 3
a, b, arrows), the ultrastructural feature of mother cells of tetraspores seems to be identical to that was observed during carposporogenesis.
So, the cytodifferentiation of carpospores and tetraspores in the heterosporic thalli of G. pistillata is comparable to that occurring in standard thalli. Its vegetative cells have potentialities to function as both auxiliary
cell/gonimoblast cell during carposporogenesis and mother cell of tetraspores during tetrasporogenesis.
Thiéry
n
s
Control
c
a
b
c
e
d
Fig. 2. a) Developing carpospore in a differentiating cystocarp (carposporogenesis. Apart from the nucleus (n), chloroplasts © and floridean starch grains (s), Golgi bodies (arrows) and cored vesicles
(arrowheads) stand out.
b-e) Reaction of carposporangia, following staining, with toluidine blue (b), PAS ©, black Sudan (d) and Thiéry’s method (e). The store cytoplasmic inclusions (floridean starch) react positively to PAS
and Thiéry’s method and do not stain with toluidine blue and black Sudan
Thiéry
Control
a
b
c
e
d
Fig. 3. a) Formation of tetraspores (tetrasporogenesis). Comparatively to fig. 2, the septa of the meiotic cruciate division (arrows) and the great aboundance of cored vesicles, must be enhanced.
b-e) Idem. Note the reaction of the inter-sporangial material to the blue toluidine staining in comparison with the carpospores (compare fig. 3b with 2 b, arrowheads).
Bar
References
1mm (Figs. 1a, b)
1 µm (Figs. 2a, e; Figs. 3a, e)
20 µm (Figs. 2b, c, d; Figs. 3 b, c, d)
1.
2.
3.
4.
Drew, K. M. (1951) Manual of Phycology. Ed. G. M. Smith
Isaac, W. E., Simons, S. M. (1954) Jl. S. Afr. Bot., 20 (1), 117
Hommersand, M., Fredericq S., Cabioch, J. (1992) Phycologia 31 (3/4), 300
Tsekos, I. (1981) J. Cell Science. 52, 71