Developmental morphology of strap-shaped gametophytes of Colysis decurrens: a new look at meristem development and function in fern gametophytes

Background and Aims

The gametophytes of most homosporous ferns are cordate–thalloid in shape. Some are strap- or ribbon-shaped and have been assumed to have evolved from terrestrial cordate shapes as an adaptation to epiphytic habitats. The aim of the present study was to clarify the morphological evolution of the strap-shaped gametophyte of microsoroids (Polypodiaceae) by precise analysis of their development.

Methods

Spores of Colysis decurrens collected in Kagoshima, Japan, were cultured and observed microscopically. Epi-illuminated micrographs of growing gametophytes were captured every 24 h, allowing analysis of the cell lineage of meristems. Light microscopy of resin-sections and scanning electron microscopy were also used.

Key Results

Contrary to previous assumptions that strap-shaped Colysis gametophytes have no organized meristem, three different types of meristems are formed during development: (1) apical-cell based – responsible for early growth; (2) marginal – further growth, including gametophyte branching; and (3) multicellular – formation of cushions with archegonia. The cushion is two or three layers thick and intermittent. The apical-cell and multicellular meristems are similar to those of cordate gametophytes of other ferns, but the marginal meristem is unique to the strap-shaped gametophyte of this fern.

Conclusions

The strap-shaped gametophytes of C. decurrens may have evolved from ancestors with a cordate shape by insertion of the marginal meristem phase between the first apical-cell-based meristem and subsequent multicellular meristem phases. Repeated retrieval of the marginal meristem at the multicellular meristem phase would result in indefinite prolongation of gametophyte growth, an ecological adaptation to epiphytic habitats.     

Local adaptation and effect of host genotype on the rate of pathogen evolution: an experimental test in a plant pathosystem

Abstract Virulence is thought to be a driving force in host–pathogen coevolution. Theoretical models suggest that virulence is an unavoidable consequence of pathogens evolving towards a high rate of intrahost reproduction. These models predict a positive correlation between the reproductive fitness of a pathogen and its level of virulence. Theoretical models also suggest that the demography and genetic structure of a host population can influence the evolution of virulence. If evolution occurs faster in pathogen populations than in host populations, the predicted result is local adaptation of the pathogen population. In our studies, we used a combination of molecular and physiological markers to test these hypotheses in an agricultural system. We isolated five strains of the fungal pathogen Mycosphaerella graminicola from each of two wheat cultivars that differed in their level of resistance to this pathogen. Each of the 10 fungal strains had distinct genotypes as indicated by different DNA fingerprints. These fungal strains were re‐inoculated onto the same two host cultivars in a field experiment and their genotype frequencies were monitored over several generations of asexual reproduction. We also measured the virulence of these 10 fungal strains and correlated it to the reproductive fitness of each fungal strain. We found that host genotypes had a strong impact on the dynamics of the pathogen populations. The pathogen population collected from the moderately resistant cultivar Madsen showed greater stability, higher genotype diversity, and smaller selection coefficients than the pathogen populations collected from the susceptible cultivar Stephens or a mixture of the two host cultivars. The pathogen collection from the mixed host population was midway between the two pure lines for most parameters measured. Our results also revealed that the measures of reproductive fitness and virulence of a pathogen strain were not always correlated. The pathogen strains varied in their patterns of local adaptation, ranging from locally adapted to locally maladapted.     

The effect of temperature and ageing on root apical meristems during seedling growth of Triticum aestivum L.: a specific effect on nucleoli

Abstract. Growth and cytology of root apical meristems of Chinese Spring wheat were studied in relation to temperature. The maximum rate of growth increased with temperature, a marked rise occurring between 10°C and 12°C. At all temperatures studied nucleolar volume increased to a maximum and then declined. The maximum nucleolar size achieved showed particular temperature sensitivity, higher volumes being attained at lower temperatures. The peak at 5°C was 70% higher than at 20°C. However, in comparison, cell and nuclear volumes were only 38% and 47% larger, respectively. Ultrastructural analysis of the nucleoli revealed a temperature-dependent relationship between the proportion of granular component and dense fibrillar component. The results are discussed in relation to the regulation of ribosome synthesis and function during growth and development at different temperatures.     

The late Cenozoic Thiaridae (Mollusca, Gastropoda, Cerithioidea) of the Albertine Rift Valley (Uganda-Congo) and their bearing on the origin and evolution of the Tanganyikan thalassoid malacofauna

The recent cerithioid malacofauna of meromictic Lake Tanganyika is unique in its degree of thalassoid convergence with marine molluscs. This is generally considered the result of a long-lasting intensive escalation and cladogenesis caused by a coevolutionary prey–predator interaction in a freshwater ecosystem with sea-like characteristics, i.e. exceptional longevity (ca. 7–12 Ma) and vast dimensions (present surface area: 32900 km², present maximum depth: 1470 m). In the Albertine Basin, ca 300 km north of the Tanganyika Trough, Palaeolake Obweruka existed during Mio-Pliocene times. In many aspects, it can be considered as a 'sister lake' of Lake Tanganyika, being also long-lived (from ca 7.5 to 2.5 Ma), extensive (surface: 27000 km²) and meromictic. Like Lake Tanganyika it belonged to the Congo catchment. Although thalassoid molluscs have been known from the Albertine deposits since the beginning of the 20th Century, previous researchers recognised only four polymorphic thiarid species, two of which were thalassoid. Detailed in situ collecting shows that the perceived low species diversity and high variability of the Obwerukan thalassoids is a phenomenon of the museum drawer. In the field the thiarid species and other molluscs are not extremely polymorphic, as formerly thought, but tend to occur in discrete morphometric packages according to stratigraphic level and geographic area. The species richness of the Palaeolake Obweruka thiarids has thus been severely underestimated, as is also the case for the Recent Tanganyikan thiarids. The present authors discern 35 species and 7 genera in the Albertine deposits (2 of which are new to science), and consider this to be a conservative estimate. In their degree of diversification and morphological escalation, the Obwerukan thiarids are thus comparable to the Tanganyika thalassoids. Arguments are presented that the thalassoids of these lakes are polyphyletic, that they are derived from the same genera (mainly Potadoma, Potadomoides and Pseudocleopatra) and that the tempo and mode of their intralacustrine evolution must have been largely similar. As for the tempo of evolution, the Albertine fossil record shows that escalation in shell morphology and ornamentation in the Thiaridae was extremely rapid, as in the Ampullariidae and Viviparidae (Van Damme & Pickford, 1995, 1999). In the Thiaridae heavily ornate shells evolved from an unornamented ancestor in two consecutive bursts, each lasting less than 0.1 million years over a total period of ca. 1 million years. The punctuated escalation in the thiarids was initiated millions of years after the lake was formed. This implies that the degree of escalation is not related to age of a lake and that the Tanganyikan thalassoids or part of them may not be particularly old, even though Lake Tanganyika itself is. As to the mode of evolution, it appears that escalation is a coevolutionary reaction to predation but that this selective pressure only triggers a reaction under specific conditions and in synergy with other environmental factors. The Obwerukan thiarid fossil record also indicates that iterative evolution of molluscan shells may be of frequent occurrence. What is considered to be a single polymorphic chronospecies, static over a great length of time, may actually be a sequence of several species that branched at different moments from the same ancestral phenotype. Finally, there is the inference that bottle-necking may be, geologically speaking, so common that it forms an essential force in macroevolution.