GENETIC EVIDENCE FOR DIPLOIDY IN EQUISETUM

Sporophytes and gametophytes of Equisetum arvense, E. laevigatum, and E. telmateia were analyzed using enzyme electrophoresis to estimate isozyme number. Despite their uniformly high chromosome numbers (2n = 216), these three species exhibited isozyme numbers typical of diploid seed plants for the enzymes AAT, ADH, ALD, GDH, [NADP]IDH, LAP, MDH, [NADP]ME, PGI, PGM, SkDH, and 6PGDH. All three species exhibited an additional isozyme for TPI. There is, therefore, no genetic evidence for low base numbers such as x = 9 and x = 12 suggested for Equisetum. Intact chloroplasts were isolated from E. arvense and the chloroplast extract compared electrophoretically to whole plant extracts. The single enzymes observed for LAP, GDH, [NADP]IDH, and [NADP]ME were absent from the chloroplast extract. Isozymes AAT-1, ALD-2, MDH-3, PGI-1, PGM-2, SkDH-2, 6PGDH-2, TPI-2, and TPI-3 were active in the chloroplast fraction; 6PGDH-1, PGI-2, PGM-1, and TPI-1 were lacking from the chloroplast fraction and were considered cytosolic. Isozymes AAT-2, MDH-1, MDH-2, MDH-4, and SkDH-1 were also lacking from the chloroplast fraction but because AAT, MDH, and SkDH have been reported from several subcellular compartments, their localization is unknown. These findings indicate that isozymes in Equisetum species are subcellularly compartmentalized as has also been demonstrated for homosporous ferns, gymnosperms, and angiosperms.     

天山森林生态系统树生地衣植物群落数量分类及其物种多样性的研究

地衣是由低等植物中的藻类与菌类中的真菌和兰细菌共生而成的一群特殊的植物。由于耐寒耐旱对生活中的养分要求不高,在养纷贫瘠、环境恶劣的高山寒漠、平原、戈壁和沙漠中地衣都能生长,因而广泛地分布和生长在不同的生态环境中。有关新疆地衣方面的研究报道集中在物种分类水平上,对于地衣群落结构方面的研究比较少。因此,我们应用多元分析中的主分量分析及聚类分析方法对分布在天山森林生态系统中树生地衣植物群落进行数量分类并对群落结构物种多样性、相似性和均匀度等群落参数进行了比较系统的研究。结果表明, 在西部天山不同森林生态系统中树生地衣群落的分布与海拔高度和树种有密切的关系, 不同海拔的不同类型的森林生态系统中分布着不同种类的树生地衣植物。 根据多元分析结果和生境的综合特征,该地区的树生地衣可划分为4种类型: 1) 茎口果粉衣+金黄茶衣群丛Chaenotheca stemonea+Candellaria aurella Association; 2)叉小孢衣+裸扁枝衣+粉唇蜈蚣衣群丛Bryoria furcellata+Evernia esorediosa+Physcia tribacoides Association; 3)亚广开梅衣+槽梅衣+疑小梅衣群丛Parmelia fertilis+Parmelia sulcata+Parmeliopsis ambigua Association; 4)睫毛黑蜈蚣衣+黑蜈蚣衣群丛Phaeophyscia ciliata+Phaeophyscia nigricans Association。其群落多样性指数以亚广开梅衣+槽梅衣+疑小梅衣群丛为最高 (1.920), 叉小孢衣+裸扁枝衣+粉唇蜈蚣衣群丛为最低 (1.562)。     

EFFECTS OF CHRONIC GAMMA IRRADIATION ON LICHEN COMMUNITIES OF A FOREST

Chronic gamma irradiation of an oak-pine forest on Long Island reduced populations of corticolous and terrestrial lichens in approximate proportion to the rate of irradiation. The decline in species diversity was approximately linearly related to logarithmic increases in daily radiation exposure. While a daily exposure of 2700 R/day was estimated to have reduced diversity to 50% in the first year, a linear extrapolation of diversity, coefficient of community and similarity curves to zero suggested that certain lichens would survive 3 years' exposure at daily rates of 5,000–10,000 R, possibly more. There was an apparent threshold at 300 R/day for radiation effects on the composition of lichen communities; changes in relative density occurred at lower exposures. The crustose forms were more resistant than foliose or fruticose forms, an observation which parallels previous observations that the most radio-resistant higher plants tend to be of short, stature.     

EVIDENCE FOR RECYCLING OF SYNAPTIC VESICLE MEMBRANE DURING TRANSMITTER RELEASE AT THE FROG NEUROMUSCULAR JUNCTION

When the nerves of isolated frog sartorius muscles were stimulated at 10 Hz, synaptic vesicles in the motor nerve terminals became transiently depleted. This depletion apparently resulted from a redistribution rather than disappearance of synaptic vesicle membrane, since the total amount of membrane comprising these nerve terminals remained constant during stimulation. At 1 min of stimulation, the 30% depletion in synaptic vesicle membrane was nearly balanced by an increase in plasma membrane, suggesting that vesicle membrane rapidly moved to the surface as it might if vesicles released their content of transmitter by exocytosis. After 15 min of stimulation, the 60% depletion of synaptic vesicle membrane was largely balanced by the appearance of numerous irregular membrane-walled cisternae inside the terminals, suggesting that vesicle membrane was retrieved from the surface as cisternae. When muscles were rested after 15 min of stimulation, cisternae disappeared and synaptic vesicles reappeared, suggesting that cisternae divided to form new synaptic vesicles so that the original vesicle membrane was now recycled into new synaptic vesicles. When muscles were soaked in horseradish peroxidase (HRP), this tracerfirst entered the cisternae which formed during stimulation and then entered a large proportion of the synaptic vesicles which reappeared during rest, strengthening the idea that synaptic vesicle membrane added to the surface was retrieved as cisternae which subsequently divided to form new vesicles. When muscles containing HRP in synaptic vesicles were washed to remove extracellular HRP and restimulated, HRP disappeared from vesicles without appearing in the new cisternae formed during the second stimulation, confirming that a one-way recycling of synaptic membrane, from the surface through cisternae to new vesicles, was occurring. Coated vesicles apparently represented the actual mechanism for retrieval of synaptic vesicle membrane from the plasma membrane, because during nerve stimulation they proliferated at regions of the nerve terminals covered by Schwann processes, took up peroxidase, and appeared in various stages of coalescence with cisternae. In contrast, synaptic vesicles did not appear to return directly from the surface to form cisternae, and cisternae themselves never appeared directly connected to the surface. Thus, during stimulation the intracellular compartments of this synapse change shape and take up extracellular protein in a manner which indicates that synaptic vesicle membrane added to the surface during exocytosis is retrieved by coated vesicles and recycled into new synaptic vesicles by way of intermediate cisternae.     

EVIDENCE FOR GENETIC HETEROZYGOSITY IN A HOMOSPOROUS FERN

Homosporous fern sporophytes from natural populations exhibited heterozygous electrophoretic patterns for several enzyme systems. Genetic tests utilizing individual gametophytes demonstrate that the observed heterozygosity is coded by alleles at single loci. This simple procedure makes it possible to distinguish segregating from fixed or phenotypic heterozygosity, previously a problem in homosporous vascular plants.     

ELECTROPHORETIC EVIDENCE FOR GENETIC DIPLOIDY IN PSILOTUM NUDUM

Psilotum nudum (2n = 104) has been considered an ancient polyploid, having resulted from repeated cycles of hybridization and allopolyploidy. However, electrophoretic analysis indicates that this species is genetically diploid despite its high chromosome number. Sixteen enzymes, encoded by 28 loci, revealed in P. nudum the number of isozymes typical of diploid seed plants. There is, therefore, no evidence of polyploid gene expression for the enzymes analyzed. These results for Psilotophyta are similar to those obtained for other lineages of homosporous pteridophytes, i.e., Arthrophyta and homosporous Microphyllophyta and Pteridophyta, all of which should be considered genetically diploid. Several hypotheses have been proposed to explain these results, most notably 1) cycles of allopolyploidy followed by massive gene silencing, and 2) initiation of these lineages with high chromosome numbers, possibly via chromosomal fission. Discrimination between these hypotheses awaits testing with molecular genetic techniques.     

EVIDENCE FOR BATESIAN MIMICRY IN A POLYMORPHIC HOVERFLY

Palatable Batesian mimics are avoided by predators because they resemble noxious or defended species. The striking resemblance of many hoverflies to noxious Hymenoptera is a “textbook” example of Batesian mimicry, but evidence that selection by predators has shaped the evolution of hoverfly patterns is weak. We looked for geographical and temporal trends in frequencies of morphs of the polymorphic hoverfly Volucella bombylans that would support the hypothesis that these morphs are Batesian mimics of different bumblebee species. The frequency of the black and yellow hoverfly morph was significantly positively related to the frequency of black and yellow bumblebees across 52 sites. Similarly, the frequency of the red‐tailed hoverfly morph was positively related to the frequency of red‐tailed bumblebees. However, the frequencies of hoverfly morphs were positively spatially autocorrelated, and after controlling for this, only one of the two common hoverfly morphs showed a significant positive relationship with its putative model. We conclude that the distribution of V. bombylans morphs probably reflects geographical variation in selection by predators resulting from differences in the frequencies of noxious bumblebee species.     

GENETIC EVIDENCE FOR ZYGOTIC MEIOSIS IN PHYTOPHTHORA CAPSICI

Variation of several traits among single-oospore isolates of the heterothallic oomycete, Phytophthora capsici, was studied to determine genetically the place of meiosis in the life cycle. Low percentages of met^– offspring were recovered from two crosses of met^– X prototrophic isolates. Arg^– cultures were also recovered from the crosses indicating the possible presence of a suppressed arg^– gene in one of the parental cultures. Three arg^– X met^– crosses yielded all four of the possible classes of offspring. Thus meiosis was either zygotic or, if gametangial, the original parental cultures must have been heterozygous at both the arg and met loci. However, when these parental cultures were crossed, only prototrophic offspring were recovered. A ratio of 3 prototrophs: 1 met^– was obtained from cross R in which two prototrophic offspring from a previous arg^– X met^– cross were mated. This indicated the presence of a suppressor of met^– and may explain the low recovery of met^– offspring from earlier crosses. From backcrosses of met^– offspring from this cross to the A² prototrophic parent, ratios close to the expected 1:1 were obtained. When the A¹ prototrophic parent of cross R (K-10) was crossed to an A² prototrophic isolate, met^– offspring were again recovered, which indicated that K-10 carried a suppressed met deficiency. In two crosses of streptomycin-resistant (S^r) X sensitive (S^s) isolates, ratio of 108 S^s: 14 S^r and 84 S^s:14 S^r were obtained. Although 1:1 ratios would have been expected if meiosis were zygotic, segregation of S^s and S^r in the F₁ generation indicated that gametangial meiosis was unlikely. Segregation of the A¹ and A² mating types in all crosses also indicated that meiosis was zygotic in P. capsici. Possible control of mating type by a single pair of alleles, by plasmids, and by more complex mechanisms is discussed in relation to the mating type ratios obtained.     

BEHAVIORAL EVIDENCE FOR HOST-RACE FORMATION IN EUROSTA SOLIDAGINIS

We report behavioral evidence that Eurosta solidaginis, a stem-galling tephritid fly, has formed host races on its two goldenrod hosts, Solidago altissima and S. gigantea. Previous work has shown that flies from each host plant differ electrophoretically at the level of host races. The two host-associated populations were truly sympatric and were frequently found on host plants of the two species growing interdigitated with each other. Each host-associated population demonstrated a strong preference for ovipuncturing its own host. The S. gigantea–associated population emerged 10 to 14 d earlier than the S. altissima–associated population, contributing to the reproductive isolation between populations. Partial reproductive isolation is also maintained by a preference for mating on the host from which the fly emerged. The populations meet the criteria established for host races, suggesting that they may be in an intermediate stage of sympatric speciation.