Kenton L. Chambers

Detection of Intraspecific Variation in Nuclear DNA Content in Microseris douglasii

A precise technique to measure DNA content in Microseris douglasii is reported. Relative absorbancy (560 nm) of Feulgen-stained spherical nuclei, arrested at the 2C DNA level, was measured microspectrophotometrically. Relative DNA contents were determined by placing epidermis of an inbred line on the same slide with epidermis from the unknown sample prior to hydrolysis and staining. The DNA values were adjusted to the internal standard; direct comparisons were made of values collected from different slides and staining batches. Differences among individual plant specimens in the 2.5%-5% range are detectable with this procedure. Intraspecific variation up to 20% in DNA content was apparent

Geographic and Ecological Distribution of Genomic DNA Content Variation in Microseris douglasii (Asteraceae)

We measured nuclear 2C DNA content (Feulgen absorbancy) of 222 plants of Microseris douglasii representing 24 geographically, ecologically, and morphologically diverse populations in California. The DNA content among plants varied more than 20% and was not correlated with morphological traits. Mean DNA contents of populations varied about 14%. For most populations, the DNA content was relatively uniform, even when the biotypes were morphologically diverse. Populations with higher DNA contents were restricted to the more mesic sites, generally with well-developed soil. A correlation between the amount of precipitation and DNA content was temporally observed within a single population near Jolon, California, over a 15-yr interim. Natural selection may be responsible, at least in part, for the observed distributional pattern of DNA content.

Genome Size Variation in Diploid Microseris Bigelovii (Asteraceae)

Variation in nuclear 2C DNA content (Feulgen absorbancy) up to 25% was detected among plants from four populations of diploid Microseris bigelovii, 2n = 18. The lower DNA values were from geographically disjunct populations growing at the latitudinal extremes of the species range. These smaller genomes may have resulted from selection for low DNA content in stressful and/or time-limited environments. These data add M. bigelovii to the short list of taxa in which intraspecific variation in DNA content has been detected.

Tropidogyne, a New Genus of Early Cretaceous Eudicots (Angiospermae) from Burmese Amber

Abstract †Tropidogyne pikei K. L. Chambers, Poinar & R. T. Buckley, representing a new genus and species, is described from an Early Cretaceous flower preserved in Burmese amber. †Tropidogyne may occupy a stem or early crown position in the phylogeny of the rosid clade. Its floral morphology, while largely plesiomorphic, can be compared with the modern family Cunoniaceae. The flower of the fossil taxon is small, bisexual, epigynous, apetalous, with five regular sepals slightly connate at the base, 10 stamens, the one preserved anther having two thecae that dehisce by longitudinal slits, an ovary of three carpels surmounted by a conspicuous disc, three short, acute styles, and a 10-ribbed inferior ovary. At the summit of each rib of the †Tropidogyne ovary is a small, darkly stained patch of tissue, interpreted here as a secretory gland.

Patterns of mean nuclear DNA content in Microseris douglasii (Asteraceae) populations

We measured DNA content of 210 plants of Microseris douglasii collected between 1980 and 1982 representing 10 populations. One major site, Parkfield-Coalinga, had a mixture of plants with a low to high DNA content in 1977, the last year of a major California drought. Collections in 1980-1982 showed an increase in mean DNA content and a reduction in variation. No differences in DNA content were detected among plants growing in different microsite habitats. Since 1978-1982 were years of generally abundant rainfall, these results are compatible with the hypothesis that increased moisture availability may be conducive to selection for higher DNA amounts. At a second site, Jolon, temporal shifts were observed from low DNA amount in the drought year of 1962 to high DNA values in 1973, and back to low DNA content in 1977. All plants of the general collections of 1980-1982 had low DNA amounts. Some high-DNA plants were detected by selectively sampling for extremely robust growth forms. The population, therefore, contained a mixture of DNA content biotypes and had not increased in mean DNA content during the improved moisture regime of 1978-1982. At a third major site, Middletown, mean DNA content was high in 1977 and decreased in 1980 and 1981. Except for one very small population, all plants growing on serpentine outcrops with poorly developed soil had low DNA. In sites where populations traversed serpentine and heavy clay substrates, all plants had similar DNA. Plants of six relatively high DNA content biotypes were grown under severely stressed growth-chamber conditions. The mean DNA content of progeny of stressed plants of each biotype was only slightly (av. = 0.8%) lower than, and the variance was similar to, that of controls.

Pappus part number in annual species ofMicroseris (Compositae, Cichoriaceae)

All North American annual species of the genusMicroseris have a five-part pappus, the one South American annual,M. pygmaea, has ten pappus parts. The pappus develops over a constant number of ten provascular bundles with or without inhibition between alternate sites of pappus development. Each natural population contains a predictable proportion of achenes with aberrant pappus part numbers. Hybridization betweenM. bigelovii (5 parts) andM. pygmaea results in F 1 and F 2 plants with many aberrant achenes. In each plant either five or ten can be shown to be the basic number with aberrant numbers following a Poisson distribution for numbers added to 5 or deleted from 10. Occasional plants show no basic number but have a random distribution of numbers about an intermediate mean. The evolutionary genetics of this character is discussed.

Evolution of microsporangium numbers in Microseris (Asteraceae. Lactuceae)

The genus Microseris contains species with disporangiate stamens and species with tetrasporangiate stamens. We determined the number of microsporangia per stamen in serial sections of heads from all 13 species of Microseris, its close relative Uropappus lindleyi, and the two allopolyploid species of Stebbinsoseris. Four Microseris species, three diploid and one tetraploid, have two microsporangia per stamen; all other species investigated have four. The most parsimonious assumption is that the disporangiate condition is derived and arose once in the evolution of Microseris. The inheritance of the number of microsporangia per stamen in crosses between M. bigelovii (disporangiate) and M. douglasii (tetrasporangiate) was determined. Segregation of microsporangium number per stamen in F2s derived from these crosses is quantitative rather than Mendelian. The average number of microsporangia per stamen in the F2 plants ranges from 2.0 to 4.0. There is a predominance of tetrasporangiate stamens in the Fl and in most F2 plants. The observed pattern of inheritance suggests a major gene with dominance and quantitative modifiers. The number of microsporangia per stamen shows little variation in flowering plants. About 90% of the species have four microsporangia, the remainder having either more or less (Endress and Stumpf, 1990). Both the constancy of the tetrasporangiate condition and deviations from it have been explained by functional constraints. Endress and Stumpf (1990) relate the occurrence of four microsporangia per stamen to the efficiency of the opening mechanism. According to these authors, deviating numbers often occur in species with specialized pollination mechanisms. For example, disporangiate anthers can occur in connection with mucilagenous pollen masses, secondary pollen presentation, cleistogamy, or opening of the anthers by means of valves. In addition, it has been suggested that in the Asteraceae, disporangiate stamens

Four additive genes determining pappus part numbers inMicroseris annual hybrid C34 (Asteraceae/Lactuceae)

Microseris strain C34 is a hybrid between the Chilean speciesM. pygmaea (10 pappus parts) and the CalifornianM. bigelovii (5 pappus parts). The F1 specimen had from 5 to 10 pappus parts per achene with an average of 6. F2, F3 and F4 plants derived from this hybrid by spontaneous selfing show segregation for the average number of pappus parts. Four segregating unlinked genes could be demonstrated, each with an allele determining 5 pappus parts from thebigelovii parent, one with an allele determining 10 pappus parts, three with null alleles from thepygmaea parent. The expected average pappus part number is the arithmetic mean of the 5- and 10-determining factors. Considerable environmental and developmental influences, both “random noise” and systematic shifts, could be demonstrated to influence the phenotypic expression. The parallel hybrid strain B87 has two 10-alleles rather than one in itspygmaea genome. The evolution of the pappus part genes ofM. pygmaea from those of abigelovii-like ancestor seems to demand the concerted (non-independent) mutation of at least two genes.

Genetic components of heterocarpy inMicroseris hybrid B87 (Asteraceae, Lactuceae)

Microseris B87 is derived from a single hybrid specimen betweenM. pygmaea with few, weakly hairy peripheral achenes and aM. bigelovii with many, strongly hairy peripheral achenes. Offspring through the F4 and F5 generations obtained by spontaneous selfing were analyzed for the segregation of quantitative and qualitative characters relating to achene dimorphism. The phenotypic effects of two previously identified unlinked genes determining the relative number of outer achenes are characterized in partially and completely homozygous sublines. We show that two morphological markers genetically linked to one of these genes are themselves regulated by the system inducing heterocarpy. Not more than two more unlinked genes are involved in the genetic basis of the heterocarpic response. The interaction of these genes in determining the heterocarpy phenotypes is discussed in the framework of a model postulating genes for a morphogen gradient across the capitulum and genes responding to this gradient.

A second marker enzyme in the genetics of pappus part numbers inMicroseris hybrid B87 (Asteraceae, Lactuceae)

CrossingMicroseris pygmaea (10 pappus parts) withM. bigelovii (5 pappus parts) results in hybrids with variable pappus part numbers between 5 and 10. Previous work has shown that a system of four additively acting genes determines the average pappus part numbers of these hybrids. In hybrid B87 two genes have a 10-determining and a 5-determining allele each, two others a 5-determining and a null (inactive or missing) allele. Genetic linkage of one of the latter with the enzyme geneEsterase-1 and the leaf shape genespatulate leaves has been demonstrated. Here we demonstrate linkage between one of the two 10-determining genes and the enzyme locusEsterase- Y/B. The genotypes in the pappus part system of many specimens can now be fully determined. This is a major advance for the analysis of the evolution of this additive polygenic system.