Ian C. Robertson

Importance of Outcrossing for Fruit Production in Slickspot Peppergrass, Lepidium Papilliferum L. (Brassicaceae)

Ian Robertson — Wed, 04 Jan 2012 13:06:48 PST

Plants with insect-mediated pollination are often assumed to be obligate outcrossers; i.e., pollen must be supplied from flowers of other individuals for pollination and subsequent fruit production. Indeed, many flowers with insect-mediated pollination exhibit incompatibility to their own pollen or have a separation in time between pollen production and maturation of the stigma on a given flower (Proctor et al. 1996). However, because the breeding systems of plants are diverse and include varying levels of outcrossing and selfing, experiments are required to determine whether pollination in a particular species occurs via outcrossing, self-pollination, or both. Here I report the results of such a study on slickspot peppergrass, Lepidium papilliferum L. (Brassicaceae), a rare mustard endemic to sagebrush-steppe habitat in southwestern Idaho.

Insect-Mediated Pollination in Slickspot Peppergrass, Lepidium Papilliferum L. (Brassicaceae), and Its Implications for Population Viability

Ian Robertson et al. — Wed, 04 Jan 2012 13:06:47 PST

Field experiments on the pollination biology of slickspot peppergrass, Lepidium papilliferum L. (Brassicaceae), a rare species endemic to microsites in sagebrush-steppe habitat in southwestern Idaho, were conducted at 2 sites from May to July 2001. Site KB contained over 10,000 plants, whereas site WG contained less than 150 plants. Insect exclusion experiments revealed that seed production in L. papilliferum is dependent on insect-mediated pollination; median percent seed set dropped from 70% to 2% when insects were excluded from flowers. A total of 25 insect families from 5 orders visited L. papilliferum flowers: 24 families at KB and 11 families at WG. Only 9 families contained more than trace amounts of pollen on their bodies: Hymenoptera-Anthophoridae, Apidae, Colletidae, Halictidae, Sphecidae, Vespidae; Coleoptera-Cerambycidae, Dermestidae; Diptera-Syrphidae. Insects from these families are likely responsible for pollination of L. papilliferum, although some may be of only minor significance due to their infrequent visits to flowers. Two of the 4 most common and pollen-laden insects found at KB, honey bees (Apis mellifera) and colletid bees, were absent or rare at WG. Three other pollen-carrying families present at KB, Sphecidae, Vespidae, and Halictidae, were not found at WG. We raise several possible explanations for this disparity in pollinator communities between sites and discuss the importance of pollinator diversity to the long term viability of L. papilliferum.

Using Automated Flight Mills to Manipulate Fat Reserves in Douglas-fir Beetles (Coleoptera: Curculionidae)

Wyatt I. Williams et al. — Wed, 14 Dec 2011 09:42:49 PST

Because current techniques for quantifying fat, the main fuel used for flight in insects, are destructive, researchers are limited to only one direct measure of fat per specimen. This limitation is problematic for studies aimed at assessing whether fat loss through flight influences subsequent behavioral activity. To overcome this problem, we used body volume, body mass, emergence day, and brood density as parameters in a multiple regression model to predict initial fat levels in female Douglas-fir beetles, Dendroctonus pseudotsugae Hopkins, on emergence from the host. The model explained 54% of the variation in fat reserves as determined by Soxhlet extraction with petroleum ether. Treatments of 30–1,380 min of flight on rotary flight mills were used to establish the relationship between flight and fat reserves. Using a model that incorporated estimated initial fat levels, as well as time spent in flight and time in nonflight activities on the flight mills, we found that 6 h of flight decreased fat by ≈50%. Flight activity and nonflight activity did not differ significantly in terms of their effect on fat reserves. Individual beetles with high initial fat content flew longer and faster on flight mills than beetles with low initial fat reserves. Our study shows how researchers can manipulate fat levels in bark beetles and other insects through flight, thereby opening the door to using these manipulations in behavioral studies.

Petal Herbivory by Chrysomelid Beetles (Phyllotreta sp.) is Detrimental to Pollination and Seed Production in Lepidium Papilliferum (Brassicaceae)

Hollie Leavitt et al. — Wed, 14 Dec 2011 09:42:45 PST

1. Insect herbivory directed at flowers can decrease fruit and seed production by decreasing the attractiveness of a damaged flower to potential pollinators, by disrupting the transfer of pollen between pollinators and stigmas, or both.

2. Effects of petal herbivory by a chrysomelid beetle (Phyllotreta sp.) on pollination and seed production in Lepidium papilliferum (Brassicaceae) were examined.

3. Under natural conditions, flowers with a hole chewed in a petal produced fruit and seed at a significantly lower rate than undamaged flowers (44% vs. 80% respectively). However, when damaged and undamaged flowers were hand pollinated, there was no significant difference in fruit or seed set (84% vs. 80% respectively).

4. Petal herbivory in L. papilliferum disrupts the effectiveness of insect-mediated pollination, but it does not physically inhibit pollination or seed production.

An Unusual Life History Strategy in Lepidium papilliferum (Brassicaceae), A Rare Mustard Endemic to Southwestern Idaho

Joshua P. White et al. — Wed, 14 Dec 2011 09:42:43 PST

Lepidium papilliferum (Brassicaceae) is a rare mustard endemic to sagebrush-steppe habitat in southwestern Idaho. The species, commonly known as slickspot peppergrass, has been described as having two life history patterns with respect to flowering — annual and biennial. Annuals germinate, flower, and die all within their first year, whereas biennials exist as vegetative rosettes in their first year, overwinter, and then reproduce and die in their second year. In this article we identify a third, albeit uncommon, pattern of flowering for L. papilliferum whereby individuals engage in limited flowering and seed production late in their first year, and then, if they survive the winter, flower and set seed again in their second year. In a study conducted at two L. papilliferum populations from June 2007 to June 2008, we found that individuals that flowered late in their first season (N = 34) suffered 59% overwinter mortality, whereas biennials that put off all reproduction until their second year (N = 200) suffered significantly less overwinter mortality at 24% (χ² = 17.094, P < 0.001). We discuss various possibilities for the adaptive significance of multiple flowering in L. papilliferum given the higher overwinter mortality for individuals that exhibit this life history strategy.

Crab Spiders Deter Insect Visitations to Slickspot Peppergrass Flowers

Ian C. Robertson et al. — Wed, 14 Dec 2011 09:42:41 PST

Insects visiting the flowers of slickspot peppergrass, Lepidium papilliferum (Brassicaceae), risk predation by crab spiders, Misumena vatia (Thomisidae). In a field study conducted at two sites in southwestern Idaho, 7.5±2.7% of L. papilliferum plants (range 0–30%, N=16 surveys of up to 40 randomly selected plants) harbored a crab spider. However, through 205 minutes of observations at plants with a spider, only 15 predation attempts were observed, with only 3 of those being successful. Despite the relatively low incidence of predation by crab spiders, an experiment revealed that the number of insects visiting L. papilliferum flowers was significantly lower at plants that harbored a crab spider than at plants free of spiders. In another experiment, floral visits increased significantly following the removal of crab spiders from individual plants. The deterrent effect of spiders was not due to a disproportionate avoidance response by certain types of insects; all insect families included in our analysis showed decreases in visitations to flowers when spiders were present, although none of these differences were statistically significant at the individual level. We found no significant change in the duration of visits to plants harboring a spider, implying either that the visitors were oblivious to the predator's presence, or that they were aware of the predator but kept their distance. Our study is one of a growing number to find a decrease in floral visits in response to predators, suggesting that the phenomenon is more widespread than was previously recognized.

Phylogenetic Relationships Among Lepidium papilliferum (L. Henderson) A. Nels. & J. F. Macbr., L. montanum Nutt., and L. davisii Rollins (Brassicaceae)

James F. Smith et al. — Wed, 24 Aug 2011 08:33:39 PDT

Previous phylogenetic analyses of Lepidium included only a few accessions of L. montanum, L. flavum, and L. fremontii to represent western North American species. Two additional species endemic to southwest Idaho have posed both taxonomic and conservation questions regarding their species status. Lepidium papilliferum was originally described as a variety of L. montanum, is morphologically similar to L. montanum, and is found in small scattered populations in southwest Idaho. The plant is restricted to specific edaphic conditions known as slick spots where high clay content creates conditions amenable to L. papilliferum, but to few other species. Resolving whether the populations of L. papilliferum merit species status distinct from L. montanum is a vexing question and phylogenetic analyses can assist in resolving this issue. Like L. papilliferum, L. davisii has specific edaphic requirements and is found in playas, areas similar to slick spots, but larger and with deeper soils. Unlike L. papilliferum, L. davisii is morphologically distinct from L. montanum and has posed less of a taxonomic quandary. Previous phylogenetic studies have shown that American species of Lepidium are derived from an ancestral allopolyploid species. In this study we have expanded previous analyses to include L. papilliferum, L. davisii, and several accessions of L. montanum along with published sequences of ITS, cpDNA, and PISTILLATA first intron. The western North American species form a monophyletic group with L. davisii sister to the remainder of the clade. Within this clade, L. papilliferum and L. fremontii are each monophyletic and sister to each other, but are imbedded within a paraphyletic L. montanum.

Relative Contributions to Seed Production by Floral Visitors of Slickspot Peppergrass, Lepidium papilliferum (Brassicaceae)

Ian C. Robertson et al. — Wed, 24 Aug 2011 08:33:37 PDT

Assessing the relative contributions to seed production made by different types of floral visitors is fundamental to understanding the evolution of floral morphology and the influence that particular pollinator taxa have on plant fitness and reproduction. This 3-year study examined the pollinator activity and the seed production in three populations of Lepidium papilliferum, a threatened mustard endemic to sagebrush-steppe habitat in southwest Idaho. Relative amounts of time visitor taxa spent foraging on flowers, visitation rates (number of flowers visited per unit time during a foraging bout), and pollination effectiveness (fruit set per single visit to a virgin flower) were recorded for each of 12 insect taxa that visited L. papilliferum flowers. Relative contributions to seed production were calculated as the product of relative interaction frequencies (the relative number of flowers visited by each taxon—the "quantity" component of pollination) and pollination effectiveness (fruit set per single visit to a virgin flower—the "quality" component of pollination). Despite the superiority of some insect taxa in terms of pollination effectiveness, estimates of relative pollinator contributions to seed production were influenced primarily by an insect taxon’s interaction frequency with flowers. Pollinator assemblages varied widely both spatially and temporally, which suggest that L. papilliferum is not under strong selective pressure to shift from its generalist pollination strategy toward greater specialization. For this threatened plant, reliance on a diverse assemblage of insect pollinators may serve as an important buffer against disruption in reproductive success caused by fluctuations in population sizes of individual pollinator taxa.

Intense Seed Predation by Harvester Ants on a Rare Mustard

Joshua P. White et al. — Thu, 14 Jul 2011 09:43:40 PDT

Seed predation can significantly restrict the reproductive output and fitness of individual plants, and its populationlevel consequences may be most severe for plants that are rare or endangered. The Owyhee harvester ant, Pogonomyrmex salinus, actively removes the fruits and seeds of slickspot peppergrass, Lepidium papilliferum, a rare mustard endemic to Idaho. We conducted a field study to investigate the extent to which P. salinus contributes to seed loss in L. papilliferum. On average, individual L. papilliferum exposed to P. salinus experienced a direct loss of > 40% of their mature fruits, whereas plants shielded from ants suffered almost no fruit loss. More than 90% of L. papilliferum seeds placed on the ground beneath plants were scavenged by ants. All L. papilliferum fruits and seeds collected by P. salinus were returned to the ants' nests and transported below ground. A search of 30 middens revealed large quantities of empty L. papilliferum fruit husks, but no intact seeds. Thus, it does not appear that the ants benefit L. papilliferum by dispersing the plant's seeds. No seed predation was detected on plants located > 20 m from a P. salinus colony. We conclude that P. salinus is the main seed predator of L. papilliferum and that in many cases the ants remove and destroy almost all of an individual plant's seeds. Seed removal of this magnitude suggests that P. salinus may significantly limit recruitment of L. papilliferum, which could lead to further decline of this rare species.

Spatial Structure and Inbreeding Depression in Slickspot Peppergrass, Lepidium Papilliferum (Brassicaceae)

Stephanie A. Billinge et al. — Tue, 01 Sep 2009 05:38:00 PDT

In plants with limited pollen and seed dispersal, populations are often spatially structured such that neighboring individuals are more closely related to one another than to distant individuals. Mating among close relatives, including selfing, may lead to a reduction in reproductive performance through the effects of prezygotic mating barriers and inbreeding depression. Studying 11 populations of slickspot peppergrass, Lepidium papilliferum (L.F. Hend.) A. Nels. and J.F. Macbr (Brassicaceae), a rare mustard endemic to southwestern Idaho, we investigated whether small populations (16–746 flowering individuals) exhibit spatial structure as previously reported for large populations (>3000 flowering individuals). Through hand-pollination experiments we found that percent fruit set increased with increasing distance between parents up to a distance of 3 m, and declined slightly but nonsignificantly at greater outcrossing distances. Self-pollinated plants produced little or no fruit. Germination of seeds from the hand-pollination experiment revealed signs of inbreeding depression in the offspring. Specifically, leaf length of developing seedlings increased significantly as a function of outcrossing distance. Total leaf number showed a similar, yet statistically nonsignificant, response to outcrossing distance. Overall, our experiments reveal spatial structuring and suggest the occurrence of inbreeding depression in small populations of L. papilliferum.

Distance Between Pollen Donor and Recipient Influences Fruiting Success in Slickspot Peppergrass, Lepidium Papilliferum

Ian C. Robertson et al. — Tue, 01 Sep 2009 05:37:58 PDT

Plant populations are often spatially structured owing to limited dispersal of pollen and seed. Mating between neighboring individuals in such populations often leads to reduced reproductive performance relative to matings between distant individuals. This response, which may be a result of inbreeding depression or prezygotic mating barriers, was investigated for slickspot peppergrass, Lepidium papilliferum L. (Brassicaceae), a rare insect-pollinated mustard endemic to sagebrush–steppe habitat in southwestern Idaho. Through hand pollination experiments we found that individual plants receiving pollen from distant sources (75–100 m and 6.5–20 km away) had significantly higher percent fruit sets than those relying on pollen from neighboring plants (<1 m>away). Self pollinated plants produced little or no fruit. These results suggest that L. papilliferum relies primarily, if not exclusively, on outcrossed pollination, and that its populations are spatially structured. Conservation efforts should therefore strive to protect sufficiently large areas of suitable habitat to ensure maintenance of genetic diversity and preserve or enhance connectivity between populations.