Modularity and selection of nectar traits in the evolution of the selfing syndrome in Ipomoea lacunosa (Convolvulaceae)

Although the evolution of the selfing syndrome often involves reductions in floral size, pollen, and nectar, few studies of selfing syndrome divergence have examined nectar. We investigate whether nectar traits have evolved independently of other floral size traits in the selfing syndrome, whether nectar traits diverged due to drift or selection, and the extent to which quantitative trait locus (QTL) analyses predict genetic correlations. We use F5 recombinant inbred lines (RILs) generated from a cross between Ipomoea cordatotriloba and I. lacunosa. We calculate genetic correlations to identify evolutionary modules, test whether traits have been under selection, identify QTLs, and perform correlation analyses to evaluate how well QTL properties reflect the genetic correlations. Nectar and floral size traits form separate genetic clusters. Directional selection has acted to reduce nectar traits in the selfing I. lacunosa. Calculations from QTL properties are consistent with observed genetic correlations. Floral trait divergence during mating system syndrome evolution reflects independent evolution of at least two evolutionary modules: nectar and floral size traits. This independence implies that adaptive change in these modules requires direct selection on both floral size and nectar traits. Our study also supports the expected mechanistic link between QTL properties and genetic correlations.

GENETIC ARCHITECTURE OF DIVERGENCE: THE SELFING SYNDROME IN IPOMOEA LACUNOSA

ABSTRACTPremise of the studyHighly selfing plant species frequently display a distinctive suite of traits termed the “selfing syndrome.” This study tests the hypothesis that these traits are grouped into correlated evolutionary modules and determines the degree of independence between such modules.MethodsWe evaluated phenotypic correlations and QTL overlaps in F2 offspring of a cross between the morning glories Ipomoea lacunosa and I. cordatotriloba and investigated how traits clustered into modules at both the phenotypic and genetic level. We then compared our findings to other QTL studies of the selfing syndrome.Key resultsIn the I. lacunosa selfing syndrome, traits group into modules that display correlated evolution within but not between modules. QTL overlap predicts phenotypic correlations, and QTLs affecting the same trait module are significantly physically clustered in the genome. The genetic architecture of the selfing syndrome varies across systems, but the pattern of stronger within-than between-module correlation is widespread.ConclusionsThe genetic architecture we observe in the selfing syndrome is consistent with a growing understanding of floral morphological integration achieved via pleiotropy in clustered traits. This view of floral evolution is consistent with resource limitation or predation driving the evolution of the selfing syndrome, but invites further research into both the selective causes of the selfing syndrome and how genetic architecture itself evolves in response to changes in mating system.

Morning glory species co-occurrence is associated with asymmetrically decreased and cascading reproductive isolation

Hybridization between species can affect the strength of the reproductive barriers that separate those species. Two extensions of this effect are: (1) the expectation that asymmetric hybridization will have asymmetric effects on reproductive barrier strength and (2) the expectation that local hybridization will affect only local reproductive barrier strength and could therefore alter within-species compatibility. We tested these hypotheses in a pair of morning glory species that exhibit asymmetric gene flow from highly selfing Ipomoea lacunosa into mixed mating I. cordatotriloba in regions where they co-occur. Because of the direction of this gene flow, we predicted that reproductive barrier strength would be more strongly affected in I. cordatotriloba than I. lacunosa. We also predicted that changes to reproductive barriers in sympatric I. cordatotriloba populations would affect compatibility with allopatric populations of that species. We tested these predictions by measuring the strength of a reproductive barrier to seed set across the species’ ranges. Consistent with our first prediction, we found that sympatric and allopatric I. lacunosa produce the same number of seeds in crosses with I. cordatotriloba, whereas crosses between sympatric I. cordatotriloba and I. lacunosa are more successful than crosses between allopatric I. cordatotriloba and I. lacunosa. This difference in compatibility appears to reflect an asymmetric decrease in the strength of the barrier to seed set in sympatric I. cordatotriloba, which could be caused by I. lacunosa alleles that have introgressed into I. cordatotriloba. We further demonstrated that changes to sympatric I. cordatotriloba have decreased its ability to produce seeds with allopatric populations of the same species, in line with our second prediction. Thus, in a manner analogous to cascade reinforcement, we suggest that introgression associated with hybridization not only influences between-species isolation but can also contribute to isolation within a species.Impact StatementBiological diversity depends on traits that prevent different species from successfully interbreeding. However, these reproductive barriers are often imperfect, leading to hybrid matings and possible genetic exchange between species where they occur together. When this happens, the reproductive barriers that separate species can themselves evolve to become stronger or weaker. Understanding the effects of hybridization on reproductive barriers is key to predicting the potential for future hybridization between species and ultimately whether hybridizing species will diverge, persist, or merge in regions where they co-occur. Here we hypothesize and show that hybridization in only one direction causes unidirectional changes to reproductive barrier strength and that geographically restricted hybridization causes local changes to barrier strength that can affect interbreeding within a species. Specifically, we found that gene flow from one species of morning glory into another likely caused a reproductive barrier to decrease in regions where they co-occur. The decreased reproductive barrier is caused by changes in only the species that received gene flow. We also found that the locally reduced barriers in the species that received gene flow affected reproductive compatibility between populations within that species. Thus, a breakdown of barriers between species can cause a build-up of barriers within a species. Our work demonstrates critical and rarely explored interactions at species boundaries.

Weed Management Programs in Grain Sorghum (Sorghum bicolor)

A field study was conducted in Arkansas over three years to evaluate various herbicide treatments, including sequential and tank-mix applications for weed control in grain sorghum (Sorghum bicolor). The herbicide treatments used were quinclorac, atrazine + dimethenamid-p, S-metolachlor followed by (fb) atrazine + dicamba, dimethenamid-p fb atrazine, S-metolachlor + atrazine fb atrazine, S-metolachlor + mesotrione, and S-metolachlor fb prosulfuron. All herbicide treatments provided excellent (90% to 100%) control of Ipomoea lacunosa, Ipomoea hederacea var. integriuscula, and Sida spinosa by 12 weeks after emergence. Quinclorac and S-metolachlor fb prosulfuron provided the lowest control of Ipomoea lacunosa, Urochloa platyphylla, Amaranthus palmeri, and Ipomoea hederacea var. integriuscula. Weed interference in the non-treated control reduced grain sorghum yield by 50% as compared to the weed-free control. S-metolachlor + mesotrione and S-metolachlor fb prosulfuron reduced sorghum yields by 1009 to 1121 kg ha−1 compared to other herbicide treatments. The five best herbicide treatments in terms of weed control and grain sorghum yield were quinclorac, atrazine + dimethenamid-p, S-metolachlor fb atrazine + dicamba, dimethenamid-p fb atrazine, and the standard treatment of S-metolachlor + atrazine fb atrazine.

Responses of Ten Weed Species to Microwave Radiation Exposure as Affected by Plant Size1

There is interest in alternative weed control methods to herbicide use, especially among those interested in organic approaches. The use of microwave radiation as a weed control method appears to be a good alternative because it does not produce chemical residues in the environment. A study was conducted to determine the impact of plant age on weed control using microwave radiation. Ten weed species, representing monocots and dicots, were selected for this study: southern crabgrass (Digitaria ciliaris (Retz.) Koeler), dallisgrass (Paspalum dilatatum Poir.), false green kyllinga (Kyllinga gracillima Miquel), fragrant flatsedge (Cyperus odoratus L.), yellow nutsedge (Cyperus esculentus L.) common ragweed (Ambrosia artemisiifolia L.), white clover (Trifolium repens L.), pitted morningglory (Ipomoea lacunosa L.), henbit (Lamium amplexicaule L.) and field bindweed (Convolvulus arvensis L.). In general, weed species become more tolerant of microwave treatments as they increased in size, as 8 to 10 week-old plants were injured less than 4 to 6 week-old plants. Most grass species regrew when treated at 90 and 180 joules.cm−2 of microwave radiation. Pitted morningglory and common ragweed showed the highest susceptibility to microwave radiation among all treated weed species. The increase in a weed's biomass over time probably increases the amount of microwave radiation necessary for heating samples to the thermal threshold required for control. Index words: Nonchemical control, microwave, weed age, weed maturity, thermal weed control. Species used in this study: southern crabgrass (Digitaria ciliaris (Retz.) Koeler); dallisgrass (Paspalum dilatatum Poir.); false green kyllinga (Kyllinga gracillima Miquel); fragrant flatsedge (Cyperus odoratus L.); yellow nutsedge (Cyperus esculentus L.); common ragweed (Ambrosia artemisiifolia L.); white clover (Trifolium repens L.); pitted morningglory (Ipomoea lacunosa L.); henbit (Lamium amplexicaule L.); field bindweed (Convolvulus arvensis L.).