RAPD mapping of three QTLs determining trichome formation in Microseris hybrid H27 (Asteraceae:Lactuceae)

Construction of a genetic linkage map in the wild plant Mimulus using RAPD and isozyme markers

As a first step to mapping quantitative trait loci for mating system differences, a genetic linkage map was generated from an interspecific backcross between Mimulus guttatus and Mimulus platycalyx. The linkage map consists of 99 RAPD and two isozyme markers. Eighty-one of these markers were mapped to 15 linkage groups, spanning 1437 contiguous centiMorgans, and covering 58% of the estimated genome. The genome length of Mimulus is estimated at 2474 +/- 35 cM; bootstrapping indicates that only ca. 40 markers are needed to give an accurate estimate of genome length. Further statistical analyses indicate that many RAPD markers cannot be ordered with certainty and that uncertain linkage groups tend to map nonlinearly even under commonly used mapping functions. Strategies for speeding up the mapping process for a wild species and possible applications of a partial linkage map in evolutionary studies are discussed. Key words : linkage map, mating system, Mimulus, RAPD.

High biological species diversity in the arctic flora

The arctic flora is considered to be impoverished, but estimates of species diversity are based on morphological assessments, which may not provide accurate counts of biological species. Here we report on crossing relationships within three diploid circumpolar plant species in the genus Draba (Brassicaceae). Although 99% of parental individuals were fully fertile, the fertility of intraspecific crosses was surprisingly low. Hybrids from crosses within populations were mostly fertile (63%), but only 8% of the hybrids from crosses within and among geographic regions (Alaska, Greenland, Svalbard, and Norway) were fertile. The frequent occurrence of intraspecific crossing barriers is not accompanied by significant morphological or ecological differentiation, indicating that numerous cryptic biological species have arisen within each taxonomic species despite their recent (Pleistocene) origin.

Polyploid origins in a circumpolar complex in Draba (Brassicaceae) inferred from cloned nuclear DNA sequences and fingerprints

Polyploid evolution has been of major importance in the arctic flora, but rarely addressed on the full circumpolar scale. Herein we study the allopolyploid Draba lactea and its close allies, which form a taxonomically intricate arctic-alpine complex including diploids, tetraploids, and hexaploids. Based on samples from the entire circumpolar area, we inferred the origins of polyploids in this complex using cloned DNA sequences from two nuclear regions (one intron from a gene encoding a second largest subunit in the RNA polymerase family, RPD2, and the ribosomal internal transcribed spacer region, ITS) and DNA fingerprints (random amplified polymorphic DNAs, RAPDs). Although D. lactea and all other polyploids examined in Draba are genetic alloploids showing fixed heterozygosity, the data obtained in the present study suggest that each of the polyploids analyzed here may have originated from a single diploid lineage: hexaploid D. lactea via tetraploid D. lactea from the D. palanderiana lineage (not from the D. fladnizensis and D. nivalis lineages as previously hypothesized), the tetraploid D. turczaninovii from the D. fladnizensis lineage, the tetraploid D. porsildii from the D. lonchocarpa lineage, and a tetraploid here named Draba spB from the D. nivalis lineage. Draba lactea has probably originated several times in the Beringian area, and it is not necessary to invoke complex origins based on a combination of different species lineages as previously suggested.

The inheritance and evolution of leaf pigmentation and pubescence in teosinte

To investigate the genetic mechanisms that underlie morphological evolution in natural populations, we employed QTL mapping to dissect the inheritance of leaf sheath characters that distinguish Chalco from Balsas teosinte. Abundant macrohairs (trichomes) and intense anthocyanin accumulation are found in Chalco teosinte sheaths whereas Balsas teosinte leaf sheaths are green and glabrous. These character states may represent adaptations to the cooler highland (Chalco) vs. warmer middle-elevation (Balsas) climates. QTL mapping in multiple populations revealed a mix of major- and minor-effect QTL affecting both sheath color (anthocyanin) and macrohair abundance. The major QTL for macrohairs accounts for 52% of the parental difference. Epistatic interactions were detected between the major-effect QTL and multiple other QTL for both traits, accounting for substantial portions of phenotypic variance. Developmental analyses suggest that regulatory program changes underlie the phenotypic differences. Sheath anthocyanin QTL are clearly associated with b1 and a3, both of which are regulators of anthocyanin biosynthesis. Our findings suggest that changes in a small number of QTL can lead to morphological evolution by modulating existing developmental programs.

QTL mapping for a trade-off between leaf and bud production in a recombinant inbred population of Microseris douglasii and M. bigelovii (Asteraceae, Lactuceae): a potential preadaptation for the colonization of serpentine soils

The different response to growth on serpentine soil is a major autecological difference between the annual asteracean species Microseris douglasii and M. bigelovii, with nearly non-overlapping distribution ranges in California. Early flowering and seed set is regarded as a crucial character contributing to escape drought and thus is strongly correlated with survival and reproductive success on serpentine as naturally toxic soil. M. bigelovii (strain C94) from non-serpentine soil produces more leaves at the expense of bud production in the first growing phase than M. douglasii (B14) from serpentine soil. A QTL mapping study for this trade-off and for other growth-related traits was performed after six generations of inbreeding (F7) from a single interspecific hybrid between B14 and C94 on plants that were grown on serpentine and alternatively on normal potting soil. The trade-off is mainly correlated with markers on one map region on linkage group 03a (lg03a) with major phenotypic effects (phenotypic variance explained [PVE] = 18.8 - 31.7 %). Plants with the M. douglasii allele in QTL-B1 (QTL-NL1) produce more buds but fewer leaves in the first 119 days on both soil types. Three modifier QTL could be mapped for bud and leaf production. In one modifier (QTL-B2 = QTL-NL4) the M. douglasii allele is again associated with more buds but fewer leaves. QTL mapped for bud set in the F6 co-localize with QTL-B1 (major QTL) and QTL-B3. Two additional QTL for leaf length and red coloration of leaves could be mapped to one map region on lg03a. Co-localization of the two QTL loci with major phenotypic effects on bud and leaf production strongly suggests that a major genetic locus controls the trade-off between the two adaptive traits. The importance of mutational changes in major genes for the adaptation to stressful environments is discussed.

QTL mapping reveals a two-step model for the evolutionary reduction of inner microsporangia within the asteracean genus Microseris

The reduction of inner (adaxial) pollen sacs (microsporangia, MS) as a diagnostic character for the three asteracean species, Microseris bigelovii, Microseris elegans and Microseris pygmaea, was analysed in an interspecific cross between Microseris douglasii and Microseris bigelovii with 4 MS and 2 MS, respectively, using the average number of MS per plant as a quantitative character. A previous QTL (Quantitative Trait Locus) analysis had revealed one major QTL (3B) and three modifier QTLs (3A, 4A, 7A) with epistatic effects only on the homozygous recessive 2 MS genotype of QTL 3B. Here we performed a bulked segregant analysis on four 2 MS and four 4 MS DNA-bulks with 407 EcoRI/ MseI AFLP-primer combinations each. In this way additional AFLP markers were mapped close to QTL 3B and QTL 3A. Three of them were converted to SCAR (Sequence Characterized Amplified region) markers. All markers were tested in natural populations of the disporangiate (2 MS) species M. bigelovii, M. elegans and M. pygmaea, and in different populations of tetrasporangiate (4 MS) M. douglasii. The marker distribution suggests that locus 3B mutated in a progenitor of the disporangiate species. QTL 3A has evolved in the 2 MS background of the major gene in the disporangiate species. Since M. pygmaea and M. bigelovii are the sister group to M. elegans, the 4 MS genotype for (markers of) QTL 3A in M. pygmaea populations is most likely due to a back mutation to the 4 MS state and could explain the slight instability of the 2 MS phenotype in this species.

A molecular–morphological approach solves taxonomic controversy in arctic Draba (Brassicaceae)

We used a combined molecular and morphological approach to solve a long-term controversy in the taxonomically complex genus Draba L. (Brassicaceae): the delimitation of Draba lactea Adams versus Draba fladnizensis Wulfen. We also tested hypotheses on the origin of D. lactea, which has been reported as hexaploid and possibly derived from the diploids D. fladnizensis, Draba nivalis Liljeblad, and (or) Draba subcapitata Simmons. In an initial analysis of large population samples of three of these autogamous species, intrapopulational isozyme variation was low or absent (mean genotypic diversity, D, was 0.10), the diploids were almost invariably homozygous, and D. lactea was highly fixed-heterozygous. In the main analysis, we examined 35–47 populations of the four tentative species from the arctic archipelago of Svalbard for variation in isozymes, random amplified polymorphic DNAs (RAPDs), and 36 morphological characters. Multivariate analyses of the RAPD data revealed four very distinct groups of multilocus phenotypes. These groups also differed in several morphological characters and corresponded to the four tentative species. The species were less differentiated at isozyme loci, in particular the diploids D. fladnizensis and D. subcapitata, but D. lactea was clearly distinguished from D. fladnizensis based on all three data sets. Contrary to most earlier suggestions, the hexaploid D. lactea was genetically more similar to D. subcapitata than to D. nivalis, but our analyses of the material from Svalbard did not give conclusive evidence on the origin of this widespread arctic hexaploid.Key words: arctic, Draba, isozymes, morphology, polyploidy, RAPDs.

The relationship between loci for mating system and fitness-related traits in Mimulus (Scrophulariaceae): a test for deleterious pleiotropy of QTLs with large effects

Loci with large phenotypic effects are generally not thought to be important in the evolution of quantitative traits because of their deleterious pleiotropic effects, yet empirical studies of such pleiotropic effects are lacking. Here I use molecular markers to test the extent of deleterious pleiotropy of quantitative trait loci (QTLs) that have large effects on mating system differences between the wild plants Mimulus guttatus and M. platycalyx (Scrophulariaceae). Six fitness-related traits, namely germination rate (GR), number of nodes (NN), number of flowers (NF), plant height (HT), above-ground biomass (WT), and flowering time (FT) were examined in a growth chamber for a backcross population between M. guttatus and M. platycalyx (with M. platycalyx as recurrent parent). Interval mapping based upon a linkage map consisting of isozyme and random amplified polymorphic DNA (RAPD) markers detected no QTL for fitness-related traits near the mating system QTLs. Single-marker analysis based upon 13 markers flanking the mating system QTLs detected three significant marker-fitness trait associations, and these associations indicate beneficial effects of mating system loci. This suggests that QTLs with large effects on mating system traits do not have significant deleterious pleiotropic effects, and that they could be important factors in adaptive evolution of Mimulus.

Genetic architecture of species differences in annual sunflowers: implications for adaptive trait introgression

Genetic architecture may profoundly influence the ability of adaptive traits to spread between species via introgressive hybridization. Here, we examine the genomic location of quantitative trait loci (QTL) associated with pollen sterility and morphological traits distinguishing two annual sunflowers, Helianthus annuus and H. debilis ssp. cucumerifolius. These species are of particular interest since they hybridize naturally, and the form of H. annuus in Texas (called ssp. texanus) is thought to have arisen through introgression. Analysis of 226 BC(1) progeny from a cross between H. annuus and H. debilis revealed 56 QTL for 15 morphological traits and 2 QTL for pollen sterility. Four morphological QTL are tightly linked (<10 cM) to one or more sterility factors and 7 are closely allied with underrepresented and presumably negatively selected chromosomal blocks. Although these 11 QTL seem unlikely to move between the species, no barrier to introgression was detected for the remaining 45 morphological QTL. In fact, due to widespread pleiotropy (or tight linkage), the introgression of just three small chromosomal blocks appears sufficient to largely recover the phenotype of ssp. texanus. Subsequent work will test for the occurrence and fitness consequences of the identified QTL in natural populations of ssp. texanus.

MOLECULAR GENETIC ANALYSIS OF TRICHOME DEVELOPMENT IN ARABIDOPSIS

Two basic questions in developmental biology are: How does a cell know when it should or should not differentiate, and once a cell is committed to differentiate, how is that process controlled? The first process regulates the arrangement or pattern of the various cell types, whereas the second makes cells functionally distinct. Together, these two processes define plant morphogenesis. Trichome development in Arabidopsis provides an excellent model to analyze these questions. First, trichome development in Arabidopsis is a relatively simple process. A single epidermal cell differentiates into a unicellular trichome. Second, this differentiation occurs in a nonrandom pattern on the plant surface. Finally, the process is amenable to genetic analysis because many mutations that affect trichome differentiation do not alter other aspects of plant development. Thus far, more than 20 genes affecting trichome development have been identified. This review examines the current state of our understanding of these genes.