Katri Kärkkäinen

GEOGRAPHICAL VARIATION IN THE INBREEDING DEPRESSION OF SCOTS PINE

The magnitude of inbreeding depression caused by recessive mutations in a population is dependent on the mutation rate and on the intensity of selection against the mutations. We studied geographical differences in the level of early inbreeding depression of Scots pine in a common garden experiment. The mean abortion rate of experimentally self‐pollinated seeds was significantly lower (75.4%) among trees that originated from northern populations (66–69°N) than among trees from more southern (60–62°N) populations (86.5%). Thus, the number of embryonic lethal equivalents was lower in the northern populations (4.5) than in the southern ones (6.9). The outcrossing rate at the mature seed stage was slightly lower in the northern populations (average 0.93) than in the southern one (0.99). The estimated selfing rate at the zygote stage varied from 0–0.28 in the populations. The reduction in the magnitude of inbreeding depression in the north may have been caused by increased levels of self‐fertilization in the northern populations. The proportion of self‐fertilized seedlings and adults was very small in all populations (F ≈ 0), indicating high inbreeding depression also in later life stages. The high level of inbreeding depression in the partially selfing Scots pine can be explained by mutation‐selection balance only if the mutation rate is high.

GENETIC BASIS OF INBREEDING DEPRESSION IN ARABIS PETRAEA

Inbreeding depression may be caused by (partially) recessive or overdominant gene action. The relative evolutionary importance of these two modes has been debated; the former mode is emphasized in the “dominance hypothesis,” the latter in the “overdominance hypothesis.” We analyzed the genetic basis of inbreeding depression in the self‐incompatible herb Arabis petraea (L.) Lam.: In the selfed progeny of twelve parental plants, we studied the proportion of chlorophyll‐deficient seedlings, the genotypic distributions of marker genes, and associations of marker genotypes with viability and quantitative traits. Early components of fitness were examined by scoring seed size, germination time, and early growth rate and by observing the proportion of chlorophyll‐deficient seedlings. Later components of fitness, flowering, and root and aboveground biomass were also measured. Marker genotypes of young seedlings were scored for 11 enzyme loci and three microsatellite markers. We found a high proportion (about 70%) of families with chlorophyll‐deficient seedlings, indicating a high mutational load. We found six significant deviations from 1:2:1 ratio at marker loci of 60 tests in seedlings, with three of these significant at the experimentwide level. Deviations from the expected ratio were assumed to be due to linked viability loci. A graphical and a Bayesian method were used to distinguish between the overdominance and dominance hypotheses. Most of the deviant segregation ratios suggested overdominance instead of recessivity of the deleterious allele. Neither the early (seed size, germination time, or early growth trait) nor the late quantitative traits (flowering, and root and aboveground biomass) showed significant linkage to markers at the experimentwide level. Presence of significant associations between markers and early viability, but lack thereof for quantitative traits expressed late, suggests either that there may be relatively low inbreeding depression in later life stages or that individual quantitative trait loci may have smaller effects than loci contributing to early viability.

Leaf Trichome Formation and Plant Resistance to Herbivory

arkk¨ ainen Leaf trichomes contribute to plant resistance against herbivory. In several plant species, the trichome density of new leaves increases after herbivore damage. Here we review the genetic basis of trichome production and the functional and adaptive significance of constitutive and induced trichome formation. We focus on leaf trichomes and their production in response to damage caused by herbivores. The genetic basis of trichome production has been explored in detail in the model species Arabidopsis thaliana. Recent comparative work indicates that the regulatory networks governing trichome development vary and that trichome production has evolved repeatedly among angiosperms. Induced trichome production has been related to increased levels of jasmonic acid in Arabidopsis, indicating a common link to other changes in resistance characteristics. Damage from insect herbivores is oftentimes negatively related to trichome production, and enhanced trichome production may thus be advantageous as it increases resistance against herbivores. There are yet few studies exploring the costs and benefits of induced trichome production in terms of plant fitness. Trichome density affects interactions with insect herbivores, but may also affect the abundance and effectiveness of predators and parasitoids feeding on herbivores, and the tolerance to abiotic stress. This suggests that an improved understanding of the functional and adaptive significance of induced trichome production requires field studies that consider the effects of trichome density on antagonistic interactions, tritrophic interactions, and plant fitness under contrasting abiotic conditions.

Why do plants abort so many developing seeds: bad offspring or bad maternal genotypes?

It has been suggested that abortion of ovules in perennials is caused partly by early acting genetic load (abortions due to ‘bad offspring’). However, it is still unclear what proportion of abortions of naturally pollinated seeds are due to early genetic load. Here we suggest that variation between maternal genotypes (abortions due to ‘bad maternal genotypes’) may be an even more important factor causing genetic abortions than early load, based on results from Scots pine. The early load is severe in Scots pine: in experimental self-pollinations on average 76% of the seeds were aborted. Comparison of naturally pollinated and experimentally cross-pollinated seeds showed that the abortion rate of naturally pollinated seeds was only slightly, and not statistically significantly, higher than that of experimentally cross-pollinated seeds (30% vs. 26.5%, respectively). Thus, although early load can be high under self-pollination in Scots pine, it does not account for a high share of abortions of naturally pollinated seeds. Instead, maternal genotype determined the seed abortion rate: in a separate experiment using an experimental population (clonal stand), 29% of the total variance in seed abortion was due to variation between maternal genotypes. We studied further whether ‘bad maternal genotypes’ could be explained by trade-offs between seed abortion and other fitness functions. Only one statistically significant genetic correlation was found, a positive association between cone production and successful seed development. Thus ‘bad maternal genotypes’ aborted a higher proportion of their seed and produced less cones than the ‘good maternal genotypes’.

The degree of early inbreeding depression determines the selfing rate at the seed stage: model and results from Pinus sylvestris (Scots pine)

Early inbreeding depression, i.e. embryonic recessive lethals, eliminates a large proportion of selfed progeny during embryo development. A model of early inbreeding depression suggested that in most conifers the variation between genotypes in the number of lethals rather than the variation in the actual rate of self-fertilization accounts for the variation between selfing rates at the seed stage. Polyembryony, the formation of several embryos per ovule in conifers, diminished the fitness cost of embryonic lethals and allowed embryo competition. We studied variation in the outcrossing rate at the seed stage in an experimental population of Scots pine. Despite extensive variation, pollen production of the trees, which is expected to predict the probability of self-fertilization, did not account for the low selfing rate variation at the seed stage. The genotypes having lowest numbers of embryonic lethals had the highest selfing rates at the seed stage. Early inbreeding depression maintains a very low selfing rate at the seed stage and masks the correlation between the rate of self-fertilization and the selfing rate at the seed stage. This is typical of most conifers and can also be common among perennial angiosperm species.

Effect of forest management on gene pools

The influence of current forest management practices on gene pools is discussed. These forest management methods range from natural regeneration to replacing local species with exotics. Genetic changes caused by drift, alterations in the mating system, and directional selection are considered. A baseline is provided by information on genetic variation and demography of natural populations. While biochemical markers are useful for monitoring broad changes in genetic diversity or levels of inbreeding, changes of adaptive characters are likely to go undetected because of the low correlation in level and pattern of variation between biochemical and adaptive characters.

Adaptive Potential of Northernmost Tree Populations to Climate Change, with Emphasis on Scots Pine (Pinus sylvestris L.)

The adaptive potential of the northernmost Pinus sylvestris L. (and other northern tree) populations is considered by examining first the current patterns of quantitative genetic adaptive traits, which show high population differentiation and clines. We then consider the postglacial history of the populations using both paleobiological and genetic data. The current patterns of diversity at nuclear genes suggest that the traces of admixture are mostly visible in mitochondrial DNA variation patterns. There is little evidence of increased diversity due to admixture between an eastern and western colonization lineage, but no signal of reduced diversity (due to sequential bottlenecks) either. Quantitative trait variation in the north is not associated with the colonizing lineages. The current clines arose rapidly and may be based on standing genetic variation. The initial phenotypic response of Scots pine in the north is predicted to be increased survival and growth. The genetic responses are examined based on quantitative genetic predictions of sustained selection response and compared with earlier simulation results that have aimed at more ecological realism. The phenotypic responses of increased growth and survival reduce the opportunity for selection and delay the evolutionary responses. The lengthening of the thermal growing period also causes selection on the critical photoperiod in the different populations. Future studies should aim at including multiple ecological and genetic factors in evaluating potential responses.

Cost of trichome production and resistance to a specialist insect herbivore in Arabidopsis lyrata.

Theory predicts that trade-offs between resistance to herbivory and other traits positively affecting fitness can maintain genetic variation in resistance within plant populations. In the perennial herb Arabidopsis lyrata, trichome production is a resistance trait that exhibits both qualitative and quantitative variation. Using a paternal half-sib design, we conducted two greenhouse experiments to ask whether trichomes confer resistance to oviposition and leaf herbivory by the specialist moth Plutella xylostella, and to examine potential genetic constraints on evolution of increased resistance and trichome density. In addition, we examined whether trichome production is induced by insect herbivory. We found strong positive genetic and phenotypic correlations between leaf trichome density and resistance to leaf herbivory, demonstrating that the production of leaf trichomes increases resistance to leaf damage by P. xylostella. Also resistance to oviposition tended to increase with increasing leaf trichome density, but genetic and phenotypic correlations were not statistically significant. Trichome density and resistance to leaf herbivory were negatively correlated genetically with plant size in the absence of herbivores, but not in the presence of herbivores. There was no evidence of increased trichome production after leaf damage by P. xylostella. The results suggest that trichome production and resistance to leaf herbivory are associated with a cost and that the direction of selection on resistance and trichome density depends on the intensity of herbivory.

Estimating numbers of embryonic lethals in conifers

Conifers have recessive lethal genes that eliminate most selfed embryos during seed development. It has been estimated that Scots pine has, on average, nine recessive lethals which act during seed development. Such high numbers are not consistent with the level of outcrossing, about 0.9–0.95, which has been observed in natural populations. Correcting for environmental mortality or using partial selfings provides significantly lower estimates of lethals. A similar discordance with numbers of lethals and observed outcrossing rates is true for other species.

Mating system analysis in a central and Northern European population of picea abies

Mating system and genetic variability were studied in a southern and a northern population of Picea abies, in Czechoslovakia and Finland. Both populations had considerable genetic variability, with expected heterozygosities of 0.17 and 0.23, respectively, at nine loci. The two distant populations were more differentiated (Gst = 0.12) than has been found in studies on Scandinavian Picea abies. Both populations had a considerable degree of partial selfing, with estimates of outcrossing of 0.83 and 0.74 in the populations from Czechoslovakia and Finland, respectively.