Genetics, experience, and host-plant preference in Eurosta solidaginis: implications for host shifts and speciation.

Host-associated mating is crucial in maintaining the partial reproductive isolation between the host races of Eurosta solidaginis (Diptera: Tephritidae), a fly that forms galls on Solidago altissima and S. gigantea. (We refer to flies reared from S. gigantea as gigantea flies and those reared from S. altissima as altissima flies.) We measured the host preference of males and females of both host races, F1 hybrids between the host races, F2, and backcrosses to both host races. Male and female altissima flies and female gigantea flies had high host fidelity, whereas male gigantea flies had low host fidelity. This result suggests that there may be gene flow between the host races due to nonassortative mating that occurs when male gigantea mate with altissima females on S. altissima. This indicates assortative-mating mechanisms in addition to host-associated mating are required to produce the partial reproductive isolation between the host races that has been observed. Nongenetic factors had no influence on host preference. Larval conditioning did not influence host preference: reciprocal F1 hybrids reared in S. altissima and S. gigantea both preferred S. gigantea. Adult experience had no impact on host preference: females preferred their natal host plant regardless of which host they encountered first as an adult. The hypothesis that maternal effects influence preferences was rejected because male and female flies did not show a consistent preference for the host plant of their mother. We also found no evidence that preference was a sex-linked trait because F1 and backcrosses to the host races with different combinations of X chromosomes from the two host races preferred S. gigantea. Our results indicate that host preference is not determined by a large number of genes because preference of hybrids did not correspond to the proportion of the genome derived from each host race. The strength of the ovipuncture preference for S. gigantea by gigantea females, the females of both reciprocal F1 hybrids, the backcross to gigantea, and F2s indicates that preference is inherited nonadditively at a limited number of loci. The F1 female hybrids, however, had a weaker host preference for S. gigantea than the pure gigantea host race, indicating that there may be incomplete dominance or modifier loci. Males had different host preference patterns than females, with individual male gigantea and male F1 hybrids usually exhibiting preference exclusively for S. gigantea or S. altissima. One hypothesis explaining the difference in host preference between males and females is that the same gene influences both female and male host preference, but it is a sex-influenced gene. Thus, males carrying the gene for S. gigantea preference have an intermediate host preference, whereas females have a strong host preference to S. gigantea. In summary, we found that the host preference that produces host-associated mating is inherited nonadditively at a relatively small number of loci on autosomal genes. This mode of inheritance meets the assumptions of models of sympatric speciation, indicating that the host races could have evolved in sympatry.

Effects of resource manipulation on the correlation between total phenolics and astringency in Douglas-fir.

The correlation between total phenolics and tannin content within a species is often considered to be suitably strong to allow researchers to assume, with some degree of confidence, that levels of one will approximately parallel the other. However, the manipulation of resource availability could lead to disproportionate changes in total phenolics and tannins and/or in the specific monomers of which these fractions are composed, thus altering the correlation between these components. In order to test this hypothesis, we examined the correlation between foliar levels of total phenolics (as measured by the ferric chloride assay) and tannins (as measured by an astringency assay) in Douglas-fir (Pseudotsuga menziesii Mirb. Franco) before and after the manipulation of nitrogen and water availability. Prior to manipulation of resources, the correlation between total phenolics and tannins was strong and highly significant (r(2)=0.869;P < 0.001). This correlation was considerably weaker and not statistically significant following resource manipulation (r(2) = 0.392; 0.20

Astringency of douglas-fir foliage in relation to phenology and xylem pressure potential.

Astringency (tannin content) of Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] foliage was examined in relation to phenology and water status. Foliage was sampled from control trees, trees on south-facing slopes, and trenched trees prior to budbreak and at periods of two, six, and 12 weeks after budbreak. Astringency of prebudbreak foliage from untrenched trees was comparable to that of mature foliage. Foliage expansion was accompanied by dilution of the tannin content. Percent relative astringency of control trees was significantly and positively related to the absolute value of predawn xylem pressure potential, while this relationship was negative for trees in the south-facing group. Coefficients of determination for the relationship between astringency and predawn xylem pressure potential were high (0.67 and 0.79 for the control and south-facing groups, respectively). Astringency of foliage from trees in the south-facing group also was affected significantly by daytime xylem pressure potential. Astringency of foliage from trees in the control and south-facing groups was not significantly related to tissue age, while that from trenched trees was significantly related only to age. Results demonstrate that water status is a better predictor of foliage astringency than is tissue age in unperturbed trees of this species and that, depending on the magnitude and/or timing of water deficits, opposite relationships between astringency and xylem pressure potential can be observed.

Tannin, nitrogen, and cell wall composition of green vs. senescent Douglas-fir foliage : Within- and between-stand differences in stands of unequal density.

Tannin, cell wall, and nitrogen composition of green foliage and needle litter of similar-aged Douglas-fir (Pseudotsuga menziesii Mirb. Franco) from two stands differing in density and crown closure were compared. Trees in the closed-canopy stand had a lower basal area growth rate than those in the open-canopy stand. Stands did not differ in wood basal area/ha or forest floor C/N ratios, but the closed-canopy stand had a significantly larger accumulation of forest floor biomass and significantly higher levels of field-extractable nitrogen and nitrogen mineralization rates. Green foliage from trees in the closed-canopy stand had significantly lower nitrogen, astringency, and lignin contents, but higher cellulose concentration than trees in the open-canopy stand. These trends, inconsistent with the inverse relationship often observed between nitrogen and polyphenol contents of foliage, may result from differences in relative resource availability in the two stands. In contrast to green foliage, needle litter from the two stands had comparable contents of nitrogen, cellulose, and lignin, but astringency was significantly higher in litter from the closed-canopy stand. It is suggested that, within the constraints imposed by site conditions, evergreens may alter the tannin composition of senescing foliage, potentially affecting herbivory and decomposition differently.