Genome size and recombination in angiosperms: a second look

Despite dramatic differences in genome size--and thus space for recombination to occur--previous workers found no correlation between recombination rate and genome size in flowering plants. Here I re-investigate these claims using phylogenetic comparative methods to test a large data set of recombination data in angiosperms. I show that genome size is significantly correlated with recombination rate across a wide sampling of species and that change in genome size explains a meaningful proportion ( approximately 20%) of variation in recombination rate. I show that the strength of this correlation is comparable with that of several characters previously linked to evolutionary change in recombination rate, but argue that consideration of processes of genome size change likely make the observed correlation a conservative estimate. And finally, although I find that recombination rate increases less than proportionally to change in genome size, several mechanistic and theoretical arguments suggest that this result is not unexpected.     

Genome size scaling through phenotype space

Background and Aims: Early observations that genome size was positively correlatedwith cell size formed the basis of hypothesized consequencesof genome size variation at higher phenotypic scales. This scalingwas supported by several studies showing a positive relationshipbetween genome size and seed mass, and various metrics of growthand leaf morphology. However, many of these studies were undertakenwith limited species sets, and often performed within a singlegenus. Here we seek to generalize the relationship between genomesize and the phenotype by examining eight phenotypic traitsusing large cross-species comparisons involving diverse assemblagesof angiosperm and gymnosperm species. These analyses are presentedin order of increasing scale (roughly equating to the numberof cells required to produce a particular phenotypic trait),following the order of: cell size (guard cell and epidermal),stomatal density, seed mass, leaf mass per unit area (LMA),wood density, photosynthetic rate and finally maximum plantheight. Scope: The results show that genome size is a strong predictor of phenotypictraits at the cellular level (guard cell length and epidermalcell area had significant positive relationships with genomesize). Stomatal density decreased with increasing genome size,but this did not lead to decreased photosynthetic rate. At higherphenotypic scales, the predictive power of genome size generallydiminishes (genome size had weak predictive power for both LMAand seed mass), except in the interesting case of maximum plantheight (tree species tend to have small genomes). There wasno relationship with wood density. The general observation thatspecies with larger genome size have larger seed mass was supported;however, species with small genome size can also have largeseed masses. All of these analyses involved robust comparativemethods that incorporate the phylogenetic relationships of species. Conclusions: Genome size correlations are quite strong at the cellular levelbut decrease in predictive power with increasing phenotypicscale. Our hope is that these results may lead to new mechanistichypotheses about why genome size scaling exists at the cellularlevel, and why nucleotypic consequences diminish at higher phenotypicscales.     

Variations in stomatal density and index: implications for palaeoclimatic reconstructions

The variation in stomatal characters in leaves from one Alnus glutinosa (L.) Gaertn. tree is analysed. Measurements were taken from over 70 sites on the abaxial surfaces of representative ‘sun’ and ‘shade’ leaves having the same insertion point. The mean values of stomatal density and index in the shade leaf were significantly lower (71 and 93%, respectively) than those for the sun leaf. Within leaves, up to 2.5-fold differences in stomatal density values were observed. Contour maps derived from the data reveal non-random trends over the leaf surface. Correlations between stomatal density, epidermal cell density and stomatal index indicate that the variation in stomatal density within leaves arose primarily from local differences in stomatal differentiation, rather than from local differences in leaf expansion. This research demonstrates that a high level of variation in stomatal characters occurs both within and between leaves. We conclude that a well-defined sampling strategy should be used when estimating stomatal characters for (tree) leaves. Furthermore, the leaf's insertion point and situation within the tree crown should be taken into account. We discuss the implications of these findings for palaeoclimatic interpretations and emphasize the need for great caution when drawing conclusions based solely on stomatal characters.     

Genome size variation and evolution in Veronica

BACKGROUND AND AIMS: The amount of DNA per chromosome set is known to be a fairly constant characteristic of a species. Its interspecific variation is enormous, but the biological significance of this variation is little understood. Some of the characters believed to be correlated with DNA amount are alpine habitat, life history and breeding system. In the present study, the aim is to distinguish between direct causal connections and chance correlation of the amount of DNA in the genus Veronica. METHODS: Estimates of DNA amount were analysed for 42 members of Veroniceae in connection with results from a phylogenetic analysis of plastid trnL-F DNA sequences and tested correlations using standard statistical tests, phylogenetically independent contrasts and a model-based generalized least squares method to distinguish the phylogenetic effect on the results. KEY RESULTS: There appears to be a lower upper limit for DNA amount in annuals than in perennials. Most DNAC-values in Veroniceae are below the mean DNA C-value for annuals in angiosperms as a whole. However, the long-debated correlation of low genome size with annual life history is not significant (P = 0.12) using either standard statistical tests or independent contrasts, but it is significant with the generalized least squares method (P < 0.01). CONCLUSIONS: The correlation of annual life history and low genome size found in earlier studies could be due to the association of annual life history and selfing, which is significantly correlated with low genome size using any of the three tests applied. This correlation can be explained by models showing a reduction in transposable elements in selfers. A significant correlation of higher genome sizes with alpine habitats was also detected.     

Ultrastructure of stomatal development in early-divergent angiosperms reveals contrasting patterning and pre-patterning

Background and Aims

Angiosperm stomata consistently possess a pair of guard cells, but differ between taxa in the patterning and developmental origin of neighbour cells. Developmental studies of phylogenetically pivotal taxa are essential as comparative yardsticks for understanding the evolution of stomatal development.

Methods

We present a novel ultrastructural study of developing stomata in leaves of Amborella (Amborellales), Nymphaea and Cabomba (Nymphaeales), and Austrobaileya and Schisandra (Austrobaileyales), representing the three earliest-divergent lineages of extant angiosperms (the ANITA-grade).

Key Results

Alternative developmental pathways occur in early-divergent angiosperms, resulting partly from differences in pre-patterning and partly from the presence or absence of highly polarized (asymmetric) mitoses in the stomatal cell lineage. Amplifying divisions are absent from ANITA-grade taxa, indicating that ostensible similarities with the stomatal patterning of Arabidopsis are superficial. In Amborella, ‘squared’ pre-patterning occurs in intercostal regions, with groups of four protodermal cells typically arranged in a rectangle; most guard-mother cells are formed by asymmetric division of a precursor cell (the mesoperigenous condition) and are typically triangular or trapezoidal. In contrast, water-lily stomata are always perigenous (lacking asymmetric divisions). Austrobaileya has occasional ‘giant’ stomata.

Conclusions

Similar mature stomatal phenotypes can result from contrasting morphogenetic factors, although the results suggest that paracytic stomata are invariably the product of at least one asymmetric division. Loss of asymmetric divisions in stomatal development could be a significant factor in land plant evolution, with implications for the diversity of key structural and physiological pathways.     

长春花叶片发育过程中气孔密度和气孔指数的动态变化

对长春花叶片近轴面和远轴面上的气孔密度和气孔指数在不同发育阶段的动态变化进行了研究.结果表明:在各个发育阶段,近轴面上的气孔以叶脉两侧居多,远轴面上的气孔则在整个叶片上均匀分布.将一个枝条上的10对真叶按发育顺序界定为10个发育阶段,即从枝条的顶端到基部,分别将第10、第9、第8……第1节位的叶片定义为第1、第2、第3...     

Ontogenetic changes in stomatal size and conductance of sunflowers

The ontogenetic changes in stomatal size, frequency and conductance (g_s) on abaxial and adaxial leaf surfaces of sunflower plants (Helianthus annuus L. Russian Mammoth) were examined under controlled environmental conditions. The stomatal frequency on the adaxial and abaxial leaf surfaces decreased with leaf ontogeny and insertion level. The ratio of adaxial to abaxial stomatal frequency did not change with leaf ontogeny and insertion level, and 42–44% of total stomata was apportioned to the adaxial surface. Ontogenetic changes in stomatal pore length were detected and increased with ontogenesis. The stomatal length of both leaf surfaces had linear relationships with leaf area. Ontogenetic changes in g_s were similar between the two surfaces. However the adaxial g_s was lower than abaxial g_s in leaves of higher insertion levels. Conductance had a linear relationship with width x frequency but not with pore area.     

Changes in stomatal frequency and size during elongation of Tsuga heterophylla needles

BACKGROUND AND AIMS: The inverse relationship between the number of stomata and atmospheric CO2 levels observed in different plant species is increasingly used for reconstructions of past CO2 concentrations. To validate this relationship, the potential influence of other environmental conditions and ontogenetical development stage on stomatal densities must be investigated as well. Quantitative data on the changes in stomatal density of conifers in relation to leaf development is reported. METHODS: Stomatal frequency and epidermal cells of Tsuga heterophylla needles during different stages of budburst were measured using computerized image analysis systems on light microscope slides. KEY RESULTS: Stomata first appear in the apical region and subsequently spread basipetally towards the needle base during development. The number of stomatal rows on a needle does not change during ontogeny, but stomatal density decreases nonlinearly with increasing needle area, until about 50 % of the final needle area. The total number of stomata on the needle increases during the entire developmental period, indicating that stomatal and epidermal cell formation continues until the needle has matured completely. CONCLUSIONS: Epidermal characteristics in developing conifer needles appear to be fundamentally different from angiosperm dicot leaves, where in general leaf expansion in the final stages is due to cell expansion rather than cell formation. The lack of further change in either stomatal density or stomatal density per millimetre needle length (the stomatal characteristic most sensitive to CO2 in conifers) in the final stages of leaf growth indicates that in conifers the stage of leaf maturation would not influence CO2 reconstructions based on stomatal density.     

Regulation of hair cell and stomatal size by a hair cell-specific peroxidase in the grass Brachypodium distachyon

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Genome size variation in pumpkin (Cucurbita sp.)

Pumpkins, a subgroup of the domesticated Cucurbita species, have been reported to range in fruit type (related to size) from miniature (<100 g) to jumbo (>273 kg). In order to obtain a wide range of fruit types it is hypothesised that all potential factors affecting fruit type must be used. One factor that is often overlooked in plant studies is genome size. In various plant species, genome size variation has been associated with characteristics such as cell size, plant size and flowering time. Such characteristics are referred to as nucleotypic parameters. In order to determine if nucleotypic selection is occurring in pumpkin, 17 varieties were analysed for genome size variation in two separate experiments. The species selected encompass the total range of fruit types reported in pumpkin. Significant nuclear DNA content variation was observed in pumpkin. There was no significant correlation between genome size and fruit type. In fact, the miniature pumpkin types were found to have the same genome size as the jumbo pumpkin types. In addition, a positive correlation between genome size and stomata length (an estimate of cell size in plants) was observed. Both the miniature and jumbo types were observed to have the smallest genome size and the smallest cell size. Thus, nucleotypic selection does appear to occur in pumpkin and appears to be involved in determining fruit type, although it may not be the only factor involved.     

Correlated evolution of genome size and seed mass

Previous investigators have identified strong positive relationships between genome size and seed mass within species, and across species from the same genus and family. Here, we make the first broad-scale quantification of this relationship, using data for 1222 species, from 139 families and 48 orders. We analyzed the relationship between genome size and seed mass using a statistical framework that included four different tests. A quadratic relationship between genome size and seed mass appeared to be driven by the large genome/seed mass gymnosperms and the many small genome size/large seed mass angiosperms. Very small seeds were never associated with very large genomes, possibly indicating a developmental constraint. Independent contrast results showed that divergences in genome size were positively correlated with divergences in seed mass. Divergences in seed mass have been more closely correlated with divergences in genome size than with divergences in other morphological and ecological variables. Plant growth form is the only variable examined thus far that explains a greater proportion of variation in seed mass than does genome size.     

Comparative body size relationships in pocket gophers and their chewing lice

In this paper, we use the method of independent contrasts to study body size relationships between pocket gophers and their chewing lice, a host-parasite system in which both host and parasite phylogcnies are well studied. The evolution of body size of chewing lice appears to be dependent only on the body size of their hosts, which confirms the 1991 findings of Harvey and Keymer. We show that there is a positive relationship between body size and hair-shaft diameter in pocket gophers, and that there is also a positive relationship between body size and head-groove width in chewing lice. Finally, we show a positive relationship between gopher hair-shaft diameter and louse head-groove width. We postulate that changes in body size of chewing lice are driven by a mechanical relationship between the parasite's head-groove dimension and the diameter of the hairs of its host. Louse species living"on larger host species may be larger simply because their hosts have thicker hairs, which requires that the lice have a wider head groove. Our study of gopher hair-shaft diameter and louse head-groove dimensions suggest that there is a 'lock-and-key' relationship between these two anatomical features.     

Size is not everything: rates of genome size evolution,not C-value,correlate with speciation in angiosperms

Angiosperms represent one of the key examples of evolutionary success, and their diversity dwarfs other land plants; this success has been linked, in part, to genome size and phenomena such as whole genome duplication events. However, while angiosperms exhibit a remarkable breadth of genome size, evidence linking overall genome size to diversity is equivocal, at best. Here, we show that the rates of speciation and genome size evolution are tightly correlated across land plants, and angiosperms show the highest rates for both, whereas very slow rates are seen in their comparatively species-poor sister group, the gymnosperms. No evidence is found linking overall genome size and rates of speciation. Within angiosperms, both the monocots and eudicots show the highest rates of speciation and genome size evolution, and these data suggest a potential explanation for the megadiversity of angiosperms. It is difficult to associate high rates of diversification with different types of polyploidy, but it is likely that high rates of evolution correlate with a smaller genome size after genome duplications. The diversity of angiosperms may, in part, be due to an ability to increase evolvability by benefiting from whole genome duplications, transposable elements and general genome plasticity.     

Terminology and classification of stomata and stomatal development—a critical survey

The literature on terminology of stomata and stomatal development is reviewed and the terminology rationalized. The classification of developmental types and of the developing cells should not be combined with the morphological classification of mature stomatal complexes. The cells involved in the development should be distinguished on the basis of their origin and position in the developing stomatal complex, and not on the basis of their future form and appearance. It is unsound to distinguish any kind of cell only on the basis of a presumed future division by which it is replaced by its two daughter cells. Development of stomata begins with the formation of a stomatal meristemoid by an unequal division of a protodermal cell. A meristemoid may divide unequally to produce a new meristemoid and a mesogene cell. Stomatal meristemoids eventually function as guard-cell mother-cells. The adjective perigene is restricted to those cells that have arisen by divisions of protodermal cells surrounding the future stoma. The undivided cells surrounding protodermal cells should be termed agene cells, and not neighbouring cells, a term which should be restricted to morphological terminology.     

Epidermal conductance, stomatal density and stomatal size among genotypes of Sorghum bicolor (L.) Moench

Abstract. The ability of a plant to survive severe water deficits depends on its ability to restrict water loss through the leaf epidermis after stomata attain minimum aperture. At this stage, the rate of water loss is regulated by the epidermal conductance (g_c). Low g_c would be a useful selection criterion to identify genotypes with enhanced survival capability. Consequently, variation in g_c among Sorghum bicolor (L.) Moench genotypes was evaluated. Since there is little conclusive evidence linking g _c with leaf waxiness, alternative hypotheses relating g _c to stomatal trails were also examined. Epidermal conductance varied from 6.3 to 17.6mmol m⁻² s⁻¹ among sorghum genotypes. It was unrelated to stomatal pore length which varied with genotype and to pore depth which was similar for all genotypes measured. However, g _c, increased with increasing stomatal density. This indicates that stomatal density plays a direct role in water loss even at very low conductances. The association of low stomatal density with low g _c is consistent with the hypothesis that at the smallest stomata aperture, water loss from the epidermis above guard cell teichodes becomes a significant source of leaf water loss. Since low g _c is directly related to crop survival under severe water deficits, it is recommended that genotypes with low g _c. be selected using the selection criterion of stomatal density.     

Wood density and vessel traits as distinct correlates of ecological strategy in 51 California coast range angiosperms

Wood density and vessel characteristics are functionally interrelated, yet they may have distinct ecological associations. In a comparative study of 51 angiosperm species ranging from chaparral shrubs to riparian trees, we examined relationships among wood density and vessel traits and their ecological correlates. Mean vessel lumen area and vessel density (number mm(-2)) varied widely (7- to 10-fold). In multivariate analyses, both vessel traits were negatively correlated with wood density, which varied more narrowly (< 2-fold). Vessel density and lumen area were inversely related across species, allowing a broad range of vessel traits within a narrow range of wood density. Phylogenetic independent contrasts indicated correlated inverse evolutionary change in vessel traits. Each trait had a distinct pattern of ecological correlation -- wood density was most strongly associated with soil water, and vessel traits showed contrasting relationships with plant height. Within a narrow range of wood density, there was significant variation in vessel traits. Given their particular ecological associations, the results suggest that wood density and vessel traits describe two distinct ecological axes.     

Genome size variation in Macaronesian angiosperms: forty percent of the Canarian endemic flora completed

Genome sizes for 127 Macaronesian endemic angiosperms from 69 genera and 32 families were estimated using propidium iodide flow cytometry. Only about 30-fold variation in 1C-values was found, ranging from 0.32 pg in Echium bonnetii to 9.52 pg in Scilla dasyantha. Taxa with very small DNA amounts (1C 1.4 pg) were the most dominant group (71.7%), whereas the frequency of other categories was much lower (18.9% and 9.4% in taxa with small (1.41–3.50 pg) and intermediate 1C-values (3.51–14.00 pg), respectively). Comparisons of average C- and Cx-values between Macaronesian endemics and non-Macaronesian representatives always revealed significantly smaller amounts in the former group at various taxonomic levels (genus, family, major phylogenetic lineage). Potential relationship between nuclear DNA content and insular burst of speciation is suggested owing to the marked prevalence of very small genomes among angiosperms that underwent rapid adaptive radiation. Merging all the genome size data on Macaronesian angiosperms available shows that this flora represents the best covered plant assemblage from the phytogeographic point of view.