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171.
The warm-season perennial switchgrass (Panicum virgatum) is a candidate bioenergy crop. To be successful, switchgrass production must be maintained on low-quality landscapes with minimal inputs while facing future climates that are expected to be more extreme and more variable. We propose that antecedent rainfall constrains how plants respond to drought, as well as subsequently recover from drought. To test this idea, we examined how six switchgrass genotypes responded to a 1-year severe drought and then recovered under normal rainfall in the following year. These plants had previously grown for 3 years under a range of dry to wet rainfall levels in a shallow-soil common garden with no fertilizer. Plants previously exposed to drought produced less biomass, and basal area after the severe drought was relieved compared to previously well-watered plants. In addition, there were legacy effects caused by plant size: plants that were larger pre-drought were more likely to survive the severe drought, and plants that were larger during the severe drought recovered more biomass, basal area, and tillers post-drought. Although genotypes differed somewhat in their responses, the size constraint was consistent across genotypes. These findings suggest that we can establish more drought-resilient switchgrass stands by, for example, planning for initial irrigation or planting during a wet year to allow plants to grow larger prior to experiencing drought. Additional studies are needed to understand whether these rainfall and size legacies persist or are transient. 相似文献
172.
Alloparental care in birds generally involves nonbreeding adults that help at nests or breeding adults that help raise young in communal nests. A less often reported form involves the amalgamation of broods, where one or more adults care for young that are not their own. We observed this phenomenon among Bristle-thighed Curlew Numenius tahitiensis broods in western Alaska during 1990–1992. Amalgamation of broods generally involved the formation of temporary and extended associations. Temporary associations were formed by the incidental convergence of broods soon after they left their nests. During this period, parents defended distinct brood-rearing areas, were antagonistic to conspecifics and remained together for less than 3 days. Extended associations formed when chicks were 1–2 weeks old. Here, parents and their broods occupied distinct, but adjacent, brood-rearing areas and moved around as a unit. Whether a brood participated in either temporary or extended associations or remained solitary appeared to depend on brood density in the immediate area and on hatching date. When chicks were 3–4 weeks old, aggregations of up to ten broods formed wherein young mixed and parents defended a common brood-rearing area. All broods (n = 48) that survived to fledging joined such aggregations. Alloparental care involved only antipredator defence and was not associated with activities such as feeding and brooding. Most female parents abandoned their broods shortly after the young could fly and when aggregations were forming. The female parent of a pair always deserted its young before or on the same day as the male parent and, in every aggregation, one or two males continued to tend young for about 5 days longer than other male parents. In most cases, adults deserted the young 2–6 days before the young departed the area when about 38 days old. Bristle-thighed Curlews also formed temporary associations with American and Pacific Golden Plover Pluvialis dominica and Pluvialis fulva, Whimbrel Numenius phaeopus, Bar-tailed Godwit Limosa lapponica, Western Sandpiper Cal-idris mauri and Long-tailed Skua Stercorarius longicaudus. Curlews and other larger bodied species commonly attack-mobbed predators together, whereas smaller bodied species generally gave alarm calls and circled the predators. For all species, the intensity of antipredator defence by attending adults gradually decreased as young became older and aggregations formed. We suggest that amalgamation of broods among Bristle-thighed Curlew enhances predator defence, aids in the process of flock formation for migrating young, and allows females and some males to desert their young earlier. 相似文献
173.
174.
Intraspecific nuclear DNA variation in Drosophila 总被引:18,自引:6,他引:12
We have summarized and analyzed all available nuclear DNA sequence
polymorphism studies for three species of Drosophila, D. melanogaster (24
loci), D. simulans (12 loci), and D. pseudoobscura (5 loci). Our major
findings are: (1) The average nucleotide heterozygosity ranges from about
0.4% to 2% depending upon species and function of the region, i.e., coding
or noncoding. (2) Compared to D. simulans and D. pseudoobscura (which are
about equally variable), D. melanogaster displays a low degree of DNA
polymorphism. (3) Noncoding introns and 3' and 5' flanking DNA shows less
polymorphism than silent sites within coding DNA. (4) X-linked genes are
less variable than autosomal genes. (5) Transition (Ts) and transversion
(Tv) polymorphisms are about equally frequent in non-coding DNA and at
fourfold degenerate sites in coding DNA while Ts polymorphisms outnumber Tv
polymorphisms by about 2:1 in total coding DNA. The increased Ts
polymorphism in coding regions is likely due to the structure of the
genetic code: silent changes are more often Ts's than are replacement
substitutions. (6) The proportion of replacement polymorphisms is
significantly higher in D. melanogaster than in D. simulans. (7) The level
of variation in coding DNA and the adjacent noncoding DNA is significantly
correlated indicating regional effects, most notably recombination. (8)
Surprisingly, the level of polymorphism at silent coding sites in D.
melanogaster is positively correlated with degree of codon usage bias. (9)
Three proposed tests of the neutral theory of DNA polymorphisms have been
performed on the data: Tajima's test, the HKA test, and the
McDonald-Kreitman test. About half of the loci fail to conform to the
expectations of neutral theory by one of the tests. We conclude that many
variables are affecting levels of DNA polymorphism in Drosophila, from
properties of nucleotides to population history and, perhaps, mating
structure. No simple, all encompassing explanation satisfactorily accounts
for the data.
相似文献
175.
Caccone A; Moriyama EN; Gleason JM; Nigro L; Powell JR 《Molecular biology and evolution》1996,13(9):1224-1232
Drosophila melanogaster belongs to a closely related group of eight species
collectively known as the melanogaster subgroup; all are native to
sub-Saharan Africa and islands off the east coast of Africa. The
phylogenetic relationships of most species in this subgroup have been well
documented; however, the three most closely related species, D. simulans,
D. sechellia, and D. mauritiana, have remained problematic from a
phylogenetic standpoint as no data set has unambiguously resolved them. We
present new DNA sequence data on the nullo and Serendipity-alpha genes and
combine them with all available nuclear DNA sequence data; the total data
encompass 12 genes and the ITS of rDNA. A methodological problem arose
because nine of the genes had information on intraspecific polymorphisms in
at least one species. We explored the effect of inclusion/exclusion of
polymorphic sites and found that it had very little effect on phylogenetic
inferences, due largely to the fact that 82% of polymorphisms are
autapomorphies (unique to one species). We have also reanalyzed our
previous DNA-DNA hybridization data with a bootstrap procedure. The
combined sequence data set and the DNA-DNA hybridization data strongly
support the sister status of the two island species, D. sechellia and D.
mauritiana. This at least partially resolves what had been a paradox of
parallel evolution in these two species.
相似文献
176.
The period (per) locus has received much attention in molecular evolution
studies because it is one of the best studied "behavioral genes" and
because it offers insight into the evolution of repetitive sequences. We
studied most of the coding region of per in Drosophila willistoni and
confirmed previously observed patterns of conservation and divergence among
distantly related species. Five regions are so highly diverged that they
cannot be aligned, whereas a region encompassing the PAS domain is very
conserved. Structural and nucleotide polymorphism patterns in the
willistoni group are not the same as those observed in previously studied
species. We sequenced the region homologous to the highly polymorphic
threonine-glycine repeat of D. melanogaster in multiple strains of D.
willistoni, as well as in other members of willistoni group, and found an
unusual amount of conservation in this region. However, the next
nonconserved region downstream in the sequence is quite variable and
polymorphic for the number of repeated glycines. The glycine codon usage is
significantly different in this glycine repeat as compared to other parts
of the gene. We were able to plot the directionality of change in the
glycine repeat region onto a phylogeny and find that the addition of
glycines is the general trend with the diversification of the willistoni
group.
相似文献
177.
178.
179.
The influence of copper exposure on predator-prey interactions in aquatic insect communities 总被引:3,自引:0,他引:3
SUMMARY. 1. Vulnerability of two species of net-spinning caddisflies (Chimarra sp. and Hydropsyche morosa) to predation by the stonefly, Paragnetina media , was significantly greater in experimental streams dosed with copper (target concentration=6 μg 1-1 ) than in control streams.
2. Results of stomach analyses showed that Hydropsychidae was the major component of stonefly diets in experimental streams and that the number of Hydropsychidae per stonefly gut was significantly greater in dosed streams than controls.
3. Since these results could not be predicted using single species bioassays, we suggest that such simple tests be supplemented with more environmentally realistic procedures that account for species interactions. 相似文献
2. Results of stomach analyses showed that Hydropsychidae was the major component of stonefly diets in experimental streams and that the number of Hydropsychidae per stonefly gut was significantly greater in dosed streams than controls.
3. Since these results could not be predicted using single species bioassays, we suggest that such simple tests be supplemented with more environmentally realistic procedures that account for species interactions. 相似文献
180.
Epidermal and mesophyll tissues of Commelina communis L. andVicia faba L. were analysed by atomic absorption spectrometryfor the major plant inorganic cations and anions (K, Na, Ca,Mg, P, NO3-N, Cl) when stomata of the leaf were open and closed.Water-soluble and residual levels of the elements were estimatedand a charge balance of the soluble fraction made. The major portion of K, Na, Cl, and P was extracted in the water-solublefraction of the epidermal and mesophyll tissues of both species.In both species the bulk of Ca remained in the insoluble residueof the epidermis whereas in mesophyll tissue it was equallydistributed be-between the two fractions in C. communis butmainly in the insoluble residue in V. faba. Magnesium was predominantlyfound in the water-soluble fraction of V. faba mesophyll tissueand distributed approximately equally between the two fractionsin the epidermal tissue. In C. communis Mg was slightly moreabundant in the water-soluble fraction of both mesophyll andepidermis. In both species no statistically significant differences inthe levels of the elements could be detected between epidermaland mesophyll tissues from leaves with open stomata and thesame tissues from leaves with closed stomata, suggesting thatthere was no major flux of ions between mesophyll and epidermisduring stomatal movements. Regardless of whether the stomata were open or closed, therewere considerably more water-soluble inorganic cations thananions present in all tissues of both species with K being themajor cation and Cl being the major anion. In V.faba and C-communis epidermis there was 4953 per cent and 56%68per cent excess cation respectively. In the mesophyll tissuethe excess cation was 6375 per cent and 75%78 per centin V.faba and C. communis respectively. When the partitioning of the levels of the elements betweenepidermis and mesophyll of a leaf is considered, except forNO3-N in both species and Na in V. faba, 20 per cent or lessof each element was present in the epidermis. 相似文献