EVOLUTION OF VEGETATIVE INCOMPATIBILITY IN FILAMENTOUS ASCOMYCETES. I. DETERMINISTIC MODELS

In ascomycetes vegetative incompatibility can prevent the somatic exchange of genetic material between conspecifics. It must occur frequently in natural populations, since in all species studied many vegetative compatibility groups (VCGs) are found. Using a population-genetic approach, this paper explores two possible selective explanations for the evolution of vegetative incompatibility in asexual fungi: selection by a nuclear parasitic gene, and selection by a harmful cytoplasmic element. In a deterministic model, assuming a random spatial distribution of VCGs in an infinitely sized population, it is found that neither of these forms of frequency-dependent selection can explain the large number of VCGs found in nature. The selective pressure for more VCGs disappears once a limited number of VCGs exist, because the frequency of compatible interactions decreases when the number of VCGs increases. In comparing the two selective explanations, selection by a cytoplasmic element seems a more plausible explanation than selection by a nuclear gene.     

COMPARATIVE ANATOMY OF THE SEED COATS OF GNETUM AND THEIR PROBABLE EVOLUTION

The seed coats of Gnetum gnemon L., G. ula Brongn., G. montanum f. parvifolium (Warb.) Mgf. and G. neglectum Bl. consist of three layers. The outer layer or sarcotesta is mostly parenchyma but contains some sclereids and fibers and a series of simple vascular bundles. The middle sclerotesta forms masses of sclereids in varying shapes and numbers, sometimes extending as a basal plate, and is usually thicker near the micropylar tube. The second layer also contains a series of small vascular bundles that reach the apex. Depending on the species, the middle layer is sometimes nearly free from the outer layer, may be partially fused with it, or completely fused to it at maturity. The innermost layer of the seed coat constitutes the endotesta which is membranous and only rarely contains sclerenchyma. It possesses a dichotomous venation system with varying degrees of anastomosing, depending upon the species. The above species show qualitative and quantitative differences in their sclerenchyma and laticifers. Seed coat anatomy may be useful in the diagnosis of some species. The trends of evolution of seed coat structure within these four species of Gnetum are discussed, and a comparison of tissue layers and vasculature with certain fossil pteridosperms is made, especially in the Trigonocarpales     

ON THE NATURE OF INTRA- AND INTERSPECIFIC INCOMPATIBILITY IN ULMUS

Efforts to introduce Dutch Elm Disease resistance into the American elm (Ulmus americana L.) through breeding with Asian elms has been hampered by sexual incompatibility. Controlled pollinations of Ulmus americana and the Siberian elm (Ulmus pumila L.) were studied in detail to gain insight into the nature of this incompatibility. Microscopic observations revealed that germination and early pollen tube growth were inhibited on the stigmatic surface following both intra- and interspecific incompatible pollinations. Both qualitative and quantitative differences in pollen inhibition on the stigmatic surface indicated that the inhibition may involve the action of an inhibitory substance. Detailed observations on callose deposition indicated that this β-1,3 glucose polymer may implement the inhibition.     

INCOMPATIBILITY OF MERISTEMATIC AND FILAMENTOUS GROWTH IN THE FERN GAMETOPHYTE

Gametophytes of Pteridium aquilinum can be maintained in red light as either 1- or 2-dimensional structures. The mode of growth realized in red light is dependent upon the activity of the meristem. An active meristem in a 2-dimensional structure will permit a continued development of that structure. A breakdown in meristematic activity results in filament formation. It is suggested that a group of actively dividing cells in some manner inhibits cell elongation and thus prevents filament formation in red light.     

THE DEVELOPMENT OF THE SEED IN ANDROGRAPHIS SERPYLLIFOLIA

Mohan Ram , H. Y. (U. Delhi, India.) The development of the seed in Andrographis serpyllifolia. Amer. Jour. Bot. 47(3) : 215—219. Illus. 1960.–Andrographis serpyllifolia, a member of the Acanthaceae, has an embryo sac with a bifurcated chalazal part. At the time of fertilization both synergids and antipodal cells disintegrate. Early in its development the endosperm is composed of 3 distinct parts: (1) a binucleate densely cytoplasmic chalazal haustorium; (2) a large binucleate micropylar haustorium; and (3) a central chamber which develops into the endosperm proper. The divisions in the central endosperm chamber are ab initio cellular. A few of the endosperm cells elongate enormously, ramify into the integument and destroy the surrounding cells. These cells have been termed secondary haustoria. Due to the unequal destruction of the integument, the endosperm assumes a ruminate condition. The mature seed is nearly naked because the seed coat is almost completely digested. The embryo has a long suspensor. The micropylar cells of the suspensor are hypertrophied and multinucleate. Contrary to Mauritzon's (1934) belief, the course of endosperm development is markedly different from that observed in Thunbergia. So far, albuminous seeds have been reported only in the subfamily Nelsonioideae. The present investigation provides a case of its occurrence in the Acanthoideae also.     

MORPHOLOGICAL RATES OF ANGIOSPERM SEED SIZE EVOLUTION

The evolution of seed size among angiosperms reflects their ecological diversification in a complex fitness landscape of life‐history strategies. The lineages that have evolved seeds beyond the upper and lower boundaries that defined nonflowering seed plants since the Paleozoic are more dispersed across the angiosperm phylogeny than would be expected under a neutral model of phenotypic evolution. Morphological rates of seed size evolution estimated for 40 clades based on 17,375 species ranged from 0.001 (Garryales) to 0.207 (Malvales). Comparative phylogenetic analysis indicated that morphological rates are not associated with the clade's seed size but are negatively correlated with the clade's position in the overall distribution of angiosperm seed sizes; clades with seed sizes closer to the angiosperm mean had significantly higher morphological rates than clades with extremely small or extremely large seeds. Likewise, per‐clade taxonomic diversification rates are not associated with the seed size of the clade but with where the clade falls within the angiosperm seed size distribution. These results suggest that evolutionary rates (morphological and taxonomic) are elevated in densely occupied regions of the seed morphospace relative to lineages whose ecophenotypic innovations have moved them toward the edges.