Robert H. Robichaux

Plant water status, hydraulic resistance and capacitance

Many excellent reviews have been written about plant water status and its measurement (e.g. Slatyer, 1967; Barrs, 1968; Boyer, 1969; Brown and van Haveren, 1972; Slavik, 1974; Turner, 1981). The reader is referred to these sources for a more complete review, particularly of the older literature. In this chapter, our major goals are to introduce the reader to the concept and measurement of plant water potential and its components, and then to discuss the consequences of gradients in these components within the plant. First, we describe the most commonly used techniques for measuring the water potential of higher plants growing under field conditions, specifically the psychrometric and pressure chamber techniques. Second, we describe methods for measuring the components of water potential, particularly turgor pressure and osmotic potential, and water content. Since the transpirational path of the plant can be regarded as a hydraulic resistor, transpirational fluxes occur only when gradients in the various components of water potential exist within the plant. Thus, the third concept that we introduce is hydraulic resistance. Techniques for its calculation are described for steady-state transpiration, when root water absorption equals shoot evaporation. Nonsteady-state flux occurs when there is a net movement of water between the transpirational path and tissues adjacent to it. Such tissues are regarded as capacitors, the final topic of our discussion.

Accelerated regulatory gene evolution in an adaptive radiation

The disparity between rates of morphological and molecular evolution remains a key paradox in evolutionary genetics. A proposed resolution to this paradox has been the conjecture that morphological evolution proceeds via diversification in regulatory loci, and that phenotypic evolution may correlate better with regulatory gene divergence. This conjecture can be tested by examining rates of regulatory gene evolution in species that display rapid morphological diversification within adaptive radiations. We have isolated homologues to the Arabidopsis APETALA3 (ASAP3/TM6) and APETALA1 (ASAP1) floral regulatory genes and the CHLOROPHYLL A/B BINDING PROTEIN9 (ASCAB9) photosynthetic structural gene from species in the Hawaiian silversword alliance, a premier example of plant adaptive radiation. We have compared rates of regulatory and structural gene evolution in the Hawaiian species to those in related species of North American tarweeds. Molecular evolutionary analyses indicate significant increases in nonsynonymous relative to synonymous nucleotide substitution rates in the ASAP3/TM6 and ASAP1 regulatory genes in the rapidly evolving Hawaiian species. By contrast, no general increase is evident in neutral mutation rates for these loci in the Hawaiian species. An increase in nonsynonymous relative to synonymous nucleotide substitution rate is also evident in the ASCAB9 structural gene in the Hawaiian species, but not to the extent displayed in the regulatory loci. The significantly accelerated rates of regulatory gene evolution in the Hawaiian species may reflect the influence of allopolyploidy or of selection and adaptive divergence. The analyses suggest that accelerated rates of regulatory gene evolution may accompany rapid morphological diversification in adaptive radiations.

Microsatellite analysis of a population crash and bottleneck in the Mauna Kea silversword, Argyroxiphium sandwicense ssp. sandwicense (Asteraceae), and its implications for reintroduction.

The Mauna Kea silversword, Argyroxiphium sandwicense ssp. sandwicense, has experienced both a severe population crash associated with an increase in alien ungulate populations on Mauna Kea, and a population bottleneck associated with reintroduction. In this paper, we address the genetic consequences of both demographic events using eight microsatellite loci. The population crash was not accompanied by a significant reduction in number of alleles or heterozygosity. However, the population bottleneck was accompanied by significant reductions in observed number of alleles, effective number of alleles, and expected heterozygosity, though not in observed heterozygosity. The effective size of the population bottleneck was calculated using both observed heterozygosities and allele frequency variances. Both methods corroborated the historical census size of the population bottleneck of at most three individuals. The results suggest that: (i) small populations, even those that result from severe reductions in historical population size and extent, are not necessarily genetically depauperate; and (ii) species reintroduction plans need to be conceived and implemented carefully, with due consideration to the genetic impact of sampling for reintroduction.

Diversity and divergence patterns in regulatory genes suggest differential gene flow in recently derived species of the Hawaiian silversword alliance adaptive radiation (Asteraceae)

The impact of gene flow and population size fluctuations in shaping genetic variation during adaptive radiation, at both the genome‐wide and gene‐specific levels, is very poorly understood. To examine how historical population size and gene flow patterns within and between loci have influenced lineage divergence in the Hawaiian silversword alliance, we have investigated the nucleotide sequence diversity and divergence patterns of four floral regulatory genes (ASAP1‐A, ASAP1‐B, ASAP3‐A, ASAP3‐B) and a structural gene (ASCAB9). Levels and patterns of molecular divergence across these five nuclear loci were estimated between two recently derived species (Dubautia ciliolata and Dubautia arborea) which are presumed to be sibling species. This multilocus analysis of genetic variation, haplotype divergence and historical demography indicates that population expansion and differential gene flow occurred subsequent to the divergence of these two lineages. Moreover, contrasting patterns of allele‐ sharing for regulatory loci vs. a structural locus between these two sibling species indicate alternative histories of genetic variation and partitioning among loci where alleles of the floral regulatory loci are shared primarily from D. arborea to D. ciliolata and alleles of the structural locus are shared in both directions. Taken together, these results suggest that adaptively radiating species can exhibit contrasting allele migration rates among loci such that allele movement at specific loci may supersede genetic divergence caused by drift and that lineage divergence during adaptive radiation can be associated with population expansion.

Population structure in the endangered Mauna Loa silversword, Argyroxiphium kauense (Asteraceae), and its bearing on reintroduction.

Reintroduction of populations of endangered species is a challenging task, involving a number of environmental, demographic and genetic factors. Genetic parameters of interest include historical patterns of genetic structure and gene flow. Care must be taken during reintroduction to balance the contrasting risks of inbreeding and outbreeding depression. The Mauna Loa silversword, Argyroxiphium kauense, has experienced a severe decline in population size and distribution in the recent past. Currently, three populations with a total of fewer than 1000 individuals remain. We measured genetic variation within and among the remnant populations using seven microsatellite loci. We found significant genetic variation remaining within all populations, probably related to the recent nature of the population impact, the longevity of the plants, and their apparent self‐incompatibility. We also found significant genetic differentiation among the populations, reinforcing previous observations of ecological and morphological differentiation. With respect to reintroduction, the results suggest that, in the absence of additional data to the contrary, inbreeding depression may not be a substantial risk as long as propagules for the founding of new populations are adequately sampled from within each source population before additional inbreeding takes place. The results further suggest that if mixing of propagules from different source populations is not required to increase within‐population genetic variation in the reintroduced populations, it may best be avoided.

Patterns of nucleotide variation in homoeologous regulatory genes in the allotetraploid Hawaiian silversword alliance (Asteraceae)

Genome‐wide duplication (polyploidization) is prevalent in a large number of eukaryotic organisms and is particularly widespread in flowering plants. Polyploid species appear to vary from their diploid progenitors in a variety of ecologically important traits, suggesting that genome duplications provide a mechanism for ecological diversification. Studies of nucleotide variation at duplicate genes that arise via polyploidization allow us to infer the evolutionary forces that act on these polyploid loci. In an effort to examine the evolutionary dynamics of homoeologous loci, molecular population genetic analyses were undertaken for duplicate regulatory genes in the allopolyploid Hawaiian silversword alliance, a premier example of adaptive radiation. The levels and patterns of nucleotide variation for the floral homeotic genes ASAPETALA1 (ASAP1) and ASAPETALA3/TM6 (ASAP3/TM6) were studied in two species representing different lineages within the Hawaiian silversword alliance: Argyroxiphium sandwicense ssp. macrocephalum and Dubautia ciliolata ssp. glutinosa. Homoeologueous copies of ASAP1 and ASAP3/TM6 show differing levels and patterns of nucleotide polymorphism. Duplicate ASAP1 copies have similar levels of nucleotide diversity and haplotype structure in both species; by contrast, duplicate ASAP3/TM6 genes display different levels and patterns of variation in D. ciliolata ssp. glutinosa. Additionally, D. ciliolata ssp. glutinosa appears to be segregating for a moderate frequency null allele in one ASAP3/TM6 homoeologue. These results suggest that differing evolutionary forces can affect duplicate loci arising from allopolyploidization.

Molecular genetic variation following a population crash in the endangered Mauna Kea silversword, Argyroxiphium sandwicense ssp. sandwicense (Asteraceae)

The endangered Mauna Kea silversword, Argyroxiphium sandwicense ssp. sandwicense (Asteraceae), has experienced a severe decline in distribution and abundance because of predation by alien ungulates. The small remnant natural population on the Mauna Kea volcano contains only 46 individuals. By contrast, the Haleakala silversword, A. sandwicense ssp. macrocephalum, consists of a large, vigorous population exceeding 60 000 individuals. Molecular genetic variation in the two populations was assessed using random amplified polymorphic DNA (RAPD) loci. Despite its severe crash in size, the Mauna Kea population did not differ significantly from the Haleakala population in the number of detectably polymorphic loci or in heterozygosity. The lack of substantial reduction in genetic variation, at least as measured with RAPD loci, suggests that the Mauna Kea population may not yet have gone through multiple generations at very small size.

Influences of gene flow on adaptive speciation in the Dubautia arborea--D. ciliolata complex.

Mechanisms of reproductive isolation during plant speciation are often unclear because distinct species often experience high levels of gene flow and hybridization. Adaptive radiations such as the Hawaiian silversword alliance (HSA) provide unique opportunities to study the interactions of selection, gene flow and isolating mechanisms during the speciation process. We examined patterns of phenotypic and genetic differentiation in Dubautia arborea and Dubautia ciliolata, two parapatric HSA taxa that show marked morphological divergence but evidence of weak molecular differentiation, in order to estimate genome‐wide differentiation and gene flow patterns. We scored 166 amplified fragment length polymorphism markers in a set of 89 plants from two populations each of D. arborea and D. ciliolata and phenotypically D. arborea‐like and D. ciliolata‐like plants from a natural hybrid zone. Analyses of population subdivision showed low levels of differentiation between the two species (FST = 0.089) and evidence that the phenotypically parental hybrid zone plants were largely of parental species rather than of hybrid origin. A Bayesian analysis of population ancestry identified a number of plants with admixed D. arborea and D. ciliolata ancestry, even in nonhybrid‐zone populations. These results suggest that genome‐wide low levels of differentiation between D. arborea and D. ciliolata are in part due to gene flow, and favour models of genic speciation and collective evolution in which gene flow has different effects on selected loci vs. nonselected genomic regions. We discuss ecological and climatic factors that may have shaped patterns of differentiation in this species complex.

Imperfect Replacement of Native Species by Non-Native Species as Pollinators of Endemic Hawaiian Plants

Native plant species that have lost their mutualist partners may require non-native pollinators or seed dispersers to maintain reproduction. When natives are highly specialized, however, it appears doubtful that introduced generalists will partner effectively with them. We used visitation observations and pollination treatments (experimental manipulations of pollen transfer) to examine relationships between the introduced, generalist Japanese White-eye (Zosterops japonicus) and 3 endemic Hawaiian plant species (Clermontia parviflora, C. montis-loa, and C. hawaiiensis). These plants are characterized by curved, tubular flowers, apparently adapted for pollination by curve-billed Hawaiian honeycreepers. Z. japonicus were responsible for over 80% of visits to flowers of the small-flowered C. parviflora and the midsize-flowered C. montis-loa. Z. japonicus-visited flowers set significantly more seed than did bagged flowers. Z. japonicus also demonstrated the potential to act as an occasional Clermontia seed disperser, although ground-based frugivory by non-native mammals likely dominates seed dispersal. The large-flowered C. hawaiiensis received no visitation by any birds during observations. Unmanipulated and bagged C. hawaiiensis flowers set similar numbers of seeds. Direct examination of Z. japonicus and Clermontia morphologies suggests a mismatch between Z. japonicus bill morphology and C. hawaiiensis flower morphology. In combination, our results suggest that Z. japonicus has established an effective pollination relationship with C. parviflora and C. montis-loa and that the large flowers of C. hawaiiensis preclude effective visitation by Z. japonicus.

Variation and macroevolution in leaf functional traits in the Hawaiian silversword alliance (Asteraceae)

Author(s): Blonder, B; Baldwin, BG; Enquist, BJ; Robichaux, RH | Abstract: