REGIONAL,LOCAL AND MICROGEOGRAPHIC ALLELE FREQUENCY VARIATION BETWEEN APPLE AND HAWTHORN POPULATIONS OF RHAGOLETIS POMONELLA IN WESTERN MICHIGAN

In the preceding study (Feder et al., 1990), we report that paired apple and hawthorn infesting populations of Rhagoletis pomonella are genetically differentiated for six allozymes. Here, we show that patterns of intra- and inter-host allele frequency variation seen for these six loci across the eastern United States are consistent on a more fine grained spatial scale in western Michigan. Malic enzyme, Aconitase-2, Mannose phosphate isomerase, and Hydroxyacid dehydrogenase all displayed significant linear relationships with latitude among five “regional” hawthorn populations sampled along a north-south transect between the cities of Cadillac and Portage, Michigan. Clines were not as evident among “regional” apple populations in western Michigan, although allele frequencies for Malic enzyme¹⁰⁰, Mannose phosphate isomerase¹⁰⁰ and Aconitase-2⁹⁵ varied with latitude among six “local” apple populations within a 60 km² area near the town of Grant. Significant allele frequency differences were observed between hawthorn and apple populations at all “regional” and “local” collecting sites analyzed in the study (a total of 20 different apple and hawthorn populations). As was the case in the geographic survey of the eastern United States, the magnitude and pattern of inter-host frequency differences at “regional” and “local” sites were a function of latitude. Host related genetic differentiation was consistent on a “microgeographic” scale as well. Allele frequencies for Malic enzyme¹⁰⁰ and Aconitase-2⁹⁵ were significantly higher over a four-year period (1984 to 1987) for flies sampled from individual hawthorn trees (N = 6) than apple trees (N = 7) within an old field (0.09-km² area) located near Grant. The fine level of genetic subdivision between hawthorn and apple populations of R. pomonella in western Michigan substantiates the existence of host associated polymorphism in the fly and supports a sympatric mode of divergence for the “apple race”.     

RADIATION DAMAGE IN APPLE SHOOT APICES

Pratt , Charlotte , John Einset , and Mohammad Zahur . (N. Y. S. Agric. Expt. Sta., Geneva.) Radiation damage in apple shoot apices. Amer. Jour. Bot. 46(7): 537–544. Illus. 1959.—Pattern of shoot growth and anatomy of the shoot apex of ‘Golden Delicious’ apple trees on ‘East Malling IX’ rootstock are compared in normal trees and those growing under chronic gamma irradiation (average doses of 17–48 r per 20-hr. day) at Brookhaven National Laboratory. Bud damage in 6 varieties of apple trees is compared. Irradiated ‘Golden Delicious’ formed lateral buds on the current year's shoot, but the following year these buds grew into spurs which failed to form a terminal bud (“budless” spurs) and enlarged to form “club tips” and “swollen spurs” in this and subsequent years. Cells with thick walls and lightly stained cytoplasm occurred in shoot apices of irradiated lateral buds in mid-June. The first tunica layer was more resistant to radiation than the inner tunica layers and the corpus; pith rib meristem was still more resistant. Inflorescence and floral meristems were rarely found, but once formed, continued development. One-year-old budless spurs had a few leaves but neither an organized apical meristem nor leaf primordia. Surface of the apex was often folded. Periderm and, later, deep-lying wound cambium developed. Expansion of pith, vascular tissue, cortex, wound cambium and periderm caused enlargement of club tips and swollen spurs. Many lateral buds from the gamma field which were propagated without irradiation in early August grew into long shoots with terminal buds. Scions removed from irradiation in November and inserted into normal trees showed lower survival and many of their shoots were budless. This suggests that the capacity for normal growth of a bud damaged by chronic irradiation is greater in mid-summer than later in the year. With reference to percentage of budless shoots, ‘Delicious,’ ‘Golden Delicious’ and ‘McIntosh’ were more sensitive to an average dose of 24 r per day and lower doses than were ‘Cox,’ ‘Macoun’ and ‘Spy.’ Symptoms of radiation damage in apple buds during the first year were similar following acute or chronic irradiation. Degree of radiation damage, as expressed by death of apical meristems, was concluded to vary with stage of development of the bud, structure of the apical meristem, and genetic constitution.     

金矮生苹果叶片气体交换参数对土壤水分的响应

 在黄土高原半干旱地区,通过测定10年生金矮生苹果（Malus pumila cv. Goldspur）叶片气体交换参数与土壤水分的定量关系,探讨了土壤水分胁迫对光合作用的影响规律,以确定苹果园节水灌溉适宜的土壤水分调控标准。结果表明：叶片的净光合速率（Pn）、蒸腾速率（Tr）、水分利用效率（WUE）、气孔导度（Gs）、细胞间隙CO2浓度（Ci）和气孔限制值（Ls）对土壤水分的变化具有明显不同的阈值反应。土壤含水量（SWC）大约在田间持水量的60%～86%范围内,Pn和Tr均保持较高的水平,小于田间持水量的60%～86%后,两者均随土壤湿度的减少而明显下降。维持较高叶片水分利用效率（WUE）的SWC约在田间持水量的50%～71%左右。当SWC小于田间持水量的48%以后,Gs和Ls明显降低,而Ci急剧增加,水分胁迫条件开始直接作用于叶肉细胞,导致光合速率下降,由气孔限制因素转变为非气孔因素。据此我们认为：在半干旱黄土高原地区,金矮生苹果园节水灌溉适宜的SWC范围大约在田间持水量的50%～71%左右,所允许的土壤水分亏缺程度为田间持水量的48%左右。     

VARIATION IN DEVELOPMENTAL CONSTRAINTS IN SIGMODON

Developmental constraints can be interpreted as factors of developmental origin responsible for covariation among measured variables. Several hypotheses have been proposed to link the possession of such constraints to subsequent evolution. Using confirmatory factor analysis, we compare developmental factors across selected taxa of cotton rats, genus Sigmodon. Three factors explain well the covariation among orofacial measurements: (1) responses to body size variation, (1) coordinated growth of traits of the occluding-tooth complex, and (3) responses to musculoskeletal interactions. Sigmodon taxa share these factors, but differ in the variance-covariance matrix of factors, and the unique variances of individual traits. Patterns of covariation among measurements of the neurocranial complex reflect responses to body size variation, and perhaps also responses to fetal brain growth. While there are no significant differences across taxa in factorpattern, variance-covariance matrix of factors, or unique variance of measured neurocranial variables, the neurocranium is only weakly constrained. We doubt that even the relatively stronger developmental constraints on the orofacial complex would prevent evolutionary divergence because differences in the variances and covariances of factors, and in levels of unique variance of individual traits can provide different opportunities for selection to act in different Sigmodon taxa.     

LATITUDINAL VARIATION IN SPECIATION MECHANISMS IN FROGS

Speciation often has a strong geographical and environmental component, but the ecological factors that potentially underlie allopatric and parapatric speciation remain understudied. Two ecological mechanisms by which speciation may occur on geographic scales are allopatric speciation through niche conservatism and parapatric or allopatric speciation through niche divergence. A previous study on salamanders found a strong latitudinal pattern in the prevalence of these mechanisms, with niche conservatism dominating in temperate regions and niche divergence dominating in the tropics, and related this pattern to Janzen's hypothesis of greater climatic zonation between different elevations in the tropics. Here, we test for latitudinal patterns in speciation in a related but more diverse group of amphibians, the anurans. Using data from up to 79 sister-species pairs, we test for latitudinal variation in elevational and climatic overlap between sister species, and evaluate the frequency of speciation via niche conservatism versus niche divergence in relation to latitude. In contrast to salamanders, we find no tendency for greater niche divergence in the tropics or for greater niche conservatism in temperate regions. Although our results support the idea of greater climatic zonation in tropical regions, they show that this climatic pattern does not lead to straightforward relationships between speciation, latitude, and niche evolution.