摘要:

This study provides data on cell division inColeochaete orbicularis, an important taxon in evolutionary theories deriving land plants from green algae. Vegetative growth in discoid species ofColeochaeteresults from marginal cell division in two planes—radial and circumferential. Like many algae and certain of the simple land plants,Coleochaeteis monoplastidic. Prior to mitosis, the single plastid migrates to a position where it will divide and be distributed into the daughter cells. Unlike monoplastidic cell division in hornworts, mosses, and lycopsids; microtubule nucleation is not intimately associated with the plastids. Instead, microtubule organization is associated with centriolar centrosomes throughout the cell cycle, as is common in algae. The cytokinetic apparatus lacks preprophase bands of microtubules, but includes typical phragmoplasts consisting of brushlike arrays of microtubules on either side of a dark zone. However, the origin and role of phragmoplasts is unusual. Phragmoplasts appear to develop among microtubules that emanate from the polar centrosomes rather than from nuclear envelopes and/or plastids. The function of phragmoplasts inColeochaeteis unclear, as the process of cytokinesis is not strictly centrifugal. Some infurrowing occurs in radial division, and cytokinesis appears to be entirely centripetal by infurrowing in circumferential division. The cortical arrays of microtubules differ from those typical of land plants in that they develop as a network in association with centrosomes after mitosis.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1994.tb15421.x

出版商:Wiley    

年代:1994

数据来源: WILEY

摘要:

Size‐dependent variations in morphological and physiological variables adduced to influence growth rate (e.g., cell surface area and volume, chlorophyllaconcentration per cell) were determined by reevaluating published data from unicellular and multicellular plants and animals. With respect to cell volume, reduced major axis regression of the available data indicated that cell surface area decreases roughly as the 0.69‐power, the concentration of Chladecreased roughly as the 0.80‐power, and cell mass decreased as the 0.77‐power. Computer simulations indicated that the scaling exponent for cell surface area was the consequence of size‐dependent variations in cell geometry and aspect ratio (i.e., cell length/width) rather than the result of geometric similitude among cells differing in size. The anisometric relation between cell mass and volume indicated that bulk cell density declines with increasing cell volume. Reanalyses of published data showed that growth rate and weight‐specific growth rate scale as the ¾‐ and negative ¼‐power, respectively, with respect to the body mass of unicellular and multicellular plants and animals. It is speculated that the anisometric relation between the growth rate and mass of unicellular plants is attributable to a “dilution” of metabolically active cellular constituents with increasing cell size in combination with the scaling of surface area with respect to volume (and therefore cell mass). It is further speculated that similar biological scalings may account for the ¾‐power rule obtained for taxonomically and ecologically diverse multicellular plants and animals.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1994.tb15422.x

出版商:Wiley    

年代:1994

数据来源: WILEY

摘要:

Aspects of floral ontogeny, breeding systems, and hybridization potential for selected members of the monophyleticPolyalthia hypoleucacomplex (Annonaceae) of Malesia were investigated. Complete intrafloral dichogamy (protogyny) was found in all five members examined. Further, these five tree species had flowers developing in several serial “cohorts,” the flowers of any one cohort maturing in synchrony resulting in complete intracohort dichogamy. Two modes of the timing of maturation of successive floral cohorts within an individual were observed. In the first (P. hypoleucaandP. sumatrana), the onset of stigma receptivity of successive cohorts was separated by 2 days, resulting in complete intercohort dichogamy within any given individual. This mode of ontogeny probably acts to reduce geitonogamy and stamen‐carpel interference within an individual. Two distinct “sets” of trees existed in these populations. The trees of one set were functionally carpellate at times when the trees of the other set were functionally staminate. Functional status for the two sets was reversed the next day. This pattern of complete intraset dichogamy with temporally shifted sets probably leads to enhanced pollen transfer by beetles between, but not within, the sets. The second mode of successive cohort maturation involved serial cohorts that achieved stigma receptivity day after day such that staminate and carpellate phase flowers were simultaneously present in a given individual. The second mode permits geitonogamy and intraindividual, interfloral stamen‐carpel interference.P. discolor, P. glauca, andP. multinervisexhibited both modes to varying degrees. In addition, autogamy was not detected forP. glauca, P. hypoleuca, andP. sumatrana.Results from manual pollination experiments are consistent with the hypothesis thatP. glaucaandP. hypoleucaare self‐incompatible, and a bidirectional hybrid cross between these two species yielded no fruit set.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1994.tb15423.x

出版商:Wiley    

年代:1994

数据来源: WILEY

摘要:

The genetic structure and mating system of an endemic Sri Lankan dipterocarp,Stemonoporus oblongifolius, was examined using allozyme analyses. Considering the small geographic distribution of the species, which is confined to the Peak Wilderness Sanctuary range, the species and population level genetic diversity are exceptionally high (He= 0.342 and 0.282, respectively). Although the populations sampled are separated by a maximum distance of only 10 kilometers, a fair amount of genetic structuring was detected (GST= 0.163). The mating system could contribute to the maintenance of a high level of genetic diversity, but in itself is insufficient to explain the high values for this study. The multilocus outcrossing rate, quantified for one population, was high (84%), with evidence for apomixis in one tree. Other factors potentially contributing to the high observed allozyme diversity in this species would be large effective population sizes, introgression of genes from sympatric species, and retention of ancient allozyme polymorphism. Gene flow among populations, estimated to be high, would have to be through pollen dispersal, since the large, resinous seeds typically fall and germinate beneath the maternal tree and are not known to be dispersed by any animal.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1994.tb15424.x

出版商:Wiley    

年代:1994

数据来源: WILEY

摘要:

The concept that pollen selection is an important force driving angiosperm evolution has received a lot of attention in recent years. In order for pollen competition to have evolutionary significance, heritable variation for pollen tube growth rate must exist. In this study, I tested the clonal repeatability, a measure of broad‐sense heritability, of in vitro pollen tube growth rate inOenothera organensis.Although I found significant phenotypic differences in pollen tube growth rate, the calculated clonal repeatability value of 9.4% indicates that heritability of pollen tube growth rate in this species is quite low. In contrast, both flower identity and plant identity had highly significant effects on pollen tube growth rate, suggesting that the environment may be more important than genotype in determining pollen performance in this species.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1994.tb15425.x

出版商:Wiley    

年代:1994

数据来源: WILEY

摘要:

Females of dioecious species usually have higher reproductive effort than males because they produce fruits in addition to flowers. Since females have higher reproductive effort, they are expected to be more negatively affected than males by low resource availability. We tested that assumption by growing females and males ofSilene latifoliaunder low levels of light, water, nitrogen, phosphorus, and potassium. Gas exchange of the sexes did not respond differently to low resource availability; higher female reproductive effort relative to males did not differentially affect their ability to assimilate carbon. However, male photosynthesis rates and stomatal conductances were slightly, but consistently, higher than those of females. The intersexual difference in photosynthesis rate may be a proximate result of reproduction if females translocate nutrients, particularly nitrogen, from their leaves to developing fruits. Alternatively (or perhaps additionally), higher male photosynthesis and stomatal conductances relative to females may be the ultimate result of sexual selection. This could be the case if 1) reproductive effort as estimated by biomass allocation is misleading and males actually invest more in reproduction than females, or 2) females experience stronger selection than males to conserve water late in the growing season, when soil moisture is likely to be low but females need to complete fruit maturation. Our results indicated that females had slightly lower leaf nitrogen but higher photosynthetic water‐use efficiency than males, so it is possible that both proximate and ultimate factors are operating simultaneously to cause lower female photosynthesis rates.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1994.tb15426.x

出版商:Wiley    

年代:1994

数据来源: WILEY

摘要:

An electrophoretic analysis of allozyme variation from 14 loci inCypripedium calceolus, C. candidum, C. arietinum, C. acaule, andC. reginaewas conducted to address evolutionary conclusions reached in prior studies on the Orchidaceae. The following specific questions were examined: 1) Do the allozyme data suggest thatC. arietinumbe transferred to the genusCriosanthes? 2) DoCypripediumpopulations contain unusually low levels of genetic variation relative to other angiosperms with comparable life histories? and 3) Is the distribution of genetic variation among populations consistent with the idea that evolutionary processes produce higher levels of among‐population differentiation in the Orchidaceae? Very low Nei's genetic identities (0.000–0.285) were found for most species comparisons; however,C. arietinumwas not the most genetically distinct taxon and should be retained in the genusCypripedium.Although mostCypripediumpopulations contain very low levels of expected heterozygosity,C. calceoluscontains unusually high levels in all populations examined. Most species examined here have below average levels of variation distributed among populations. This result indicates that evolutionary processes produce relatively lower, not higher, levels of genetic differentiation amongCypripediumpopulations compared to other flowering plants. Historical events that could have influenced the observed genetic patterns are discussed.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1994.tb15427.x

出版商:Wiley    

年代:1994

数据来源: WILEY

摘要:

Two sister species inDisasectionDisawere studied to evaluate the effect that minor differences in floral color, shape, and scent have on pollination.D. atricapillaandD. bivalvatahave overlapping distributional ranges, occupy similar habitats, flower at the same time, and often occur sympatrically. Observations at several sites indicate that each species is pollinated almost exclusively by male wasps.D. atricapillais pollinated byPodalonia canescens(Sphecidae), whileD. bivalvatais pollinated mainly byHemipepsis hilarisSmith (Pompilidae). Both wasp genera appear to exhibit mate‐seeking behavior when approaching and visiting flowers. This together with the absence of a floral reward suggests thatD. atricapillaandD. bivalvataare pollinated through sexual deception. Pollination success often appears low, but because of long floral life spans, overall fruiting success is generally high. The occasional presence of hybrids in areas of sympatry is attributed to secondary beetle pollinators. The rarity of these hybrids indicates that prefertilization barriers between the species are usually strong. This is the first report of pollination through sexual deception for southern Africa.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1994.tb15428.x

出版商:Wiley    

年代:1994

数据来源: WILEY

摘要:

In plants capable of both self‐fertilization and outcrossing, the selfing rate depends on the proportion of self pollen in pollen loads and on the relative postpollination success of self pollen in siring offspring. While the composition of pollen loads is subject to unpredictable variation, paternity success of self vs. outcross pollen following pollen deposition may be controlled by maternal plants. This study examined postpollination paternity success inClarkia gracilisssp.sonomensis, in which deposition of self pollen is common. Pure loads of self and outcross pollen produced similar numbers of mature seeds, but equal mixtures of self and outcross pollen yielded more than three times as many outcrossed offspring as selfed offspring. The finding that the paternity success of self pollen depends on whether it is in competition with outcross pollen helps to explain an earlier finding that the selfing rate in experimental populations was highest when pollinator activity was lowest. Cryptic self‐incompatibility allows paternity by self pollen when outcross pollen is unavailable.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1994.tb15429.x

出版商:Wiley    

年代:1994

数据来源: WILEY

摘要:

We inferred Lloyd's modes of selfing in a natural population of the common monkeyflower,Mimulus guttatus.Estimates were obtained using floral manipulations combined with seed counts and isozyme analyses of selfing rates. Of the 25% selfing estimated from isozyme markers, about one‐half was competing, about one‐third was geitonogamous, and at least one‐fifth (perhaps twice this) was due to biparental inbreeding. Estimates of prior and delayed selfing were small and did not significantly differ from zero. These results were obtained using plants with the characteristic pair of open flowers at an inflorescence node. The second‐opening flower showed twice the rate of selfing, presumably because of protogynous‐based geitonogamy differences. Solitary‐flowered plants, which have smaller flowers but no geitonogamy, showed about 50% selfing, consisting of about equal components of competing selfing and biparental inbreeding. While geitonogamy and biparental inbreeding might be unavoidable by‐products of adaptations for outcrossing, competing selfing is subject to more direct natural selection and warrants adaptive explanations.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1994.tb15430.x

出版商:Wiley    

年代:1994

数据来源: WILEY