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Genetic and reproductive factors contributing to the invasiveness of Cortaderia jubata and C. selloana in California. (03XN018)

Program

Exotic Pests and Diseases Research Program

Principal
investigators

M.A. Jasieniuk, Vegetable Crops, UC Davis
J.M. DiTomaso, Vegetable Crops, UC Davis

Host/habitat

Wildland

Pest

Jubatagrass Cortaderia jubata; Pampasgrass Cortaderia selloana

Discipline

Weed Science

Review
panel

Natural Systems

Start year (duration)

2003 (Three Years)

Objectives

Quantify the genetic structure of wild populations of Cortaderia jubata and C. selloana and use this information to trace the species' spread in California.

Identify ornamental plantings, cultivars sold by nurseries, and/or wild coastal populations of Cortaderia that are sources of recent invasions into inland habitats.

Quantify sex ratios of C. selloana in ornamental plantings and wild populations, and evaluate the effect of population sex ratio on the fitness of female and hermaphroditic plants.

Project
Summary

Cortaderia jubata and C. selloana are among the most invasive wildland exotic pests in California. Since their introduction in the late 1800s, their distribution in natural environments has largely been restricted to coastal habitats. Recently, however, both species, but in particular C. selloana, have been observed to be colonizing drier, inland habitats. We propose to identify genetic and reproductive factors contributing to the increased invasiveness of Cortaderia. Nuclear and chloroplast DNA markers will be used to quantify population genetic structure, and evaluate the role of genetic diversity, multiple introductions, and pollen dispersal. For C. selloana, a gynodioecious species currently grown as an ornamental, the invasiveness of different cultivars, and the effect of varying population sex ratios on the fitness of female and hermaphroditic plants, will also be investigated.

Final report

Our studies of pampas grass indicated that invasive populations in California are spreading as a result of dispersal from cultivated landscape plantings. The major horticultural source of landscape plantings contributing to invasive populations was unnamed pampas grass, which is sold without cultivar designation by nurseries and garden outlets. However, additional research is required to determine whether the invasiveness of unnamed pampas grass is an inherent quality of the selection or if it reflects the selection's abundance in California landscape plantings. In addition to unnamed pampas grass, we also found evidence of the cultivated gene pool that includes the two named cultivars 'Ivory Feathers' and 'Monvin' in invasive populations. Our results strongly point to dispersal from landscape plantings as causing the spread of invasive pampas grass in California. Management efforts that target the eradication of landscape plantings or their seed and pollen production are likely to be highly effective in controlling invasive populations, as well as preventing further invasive spread of pampas grass.

Our studies of jubata grass indicated that all invasive plants sampled in California consisted of a single clone. The clone is likely to have come from southern Ecuador and is identical to invasive jubata grass found in Hawaii and New Zealand. Widespread geographic distribution of a single clone is consistent with a single genetic origin and the lack of sexual reproduction reported for this species. The genetic uniformity of invasive jubata grass in California suggests that classical biological control of the species could be successful. A lack of genetic diversity simplifies identification of native source populations to search for natural enemies, as well as reduces the variability in establishment and effectiveness of biological control agents that is often associated with genetic variation in the host plant.

Third-year
progress

Jubata grass and pampas grass are among the most invasive wildland weeds in California. To trace their geographical spread across the state and identify the reproductive factors contributing to spread, we used molecular markers to determine the genetic structure of invasive populations and the identity of native or cultivated plants that gave rise to invasives. We determined that all invasive plants of jubata grass in California consist of a single clonal genotype that is likely to have come from southern Ecuador and is identical to invasive jubata grass in Hawaii and New Zealand. In contrast, our studies indicate that invasive populations of pampas grass in California resulted from repeated introductions from multiple sources and most likely originated from landscape plantings. Two cultivated gene pools were the most abundant in landscape plantings and invasive populations. One consisted of generic pampas grass without cultivar designation. The second included the two named cultivars 'Ivory Feathers' and 'Monvin' and the generic pampas grass. During the final year of our study, we will focus our efforts on mapping and characterizing landscape plantings to determine whether cultural practices may be modified to ensure ornamental plantings do not produce seed.

Second-year
progress

To identify the origins and trace the spread of invasive pampas grass and jubata grass in California, we sampled leaf tissue of plants in 29 wild populations of pampas grass and 23 wild populations of jubata grass across the state, and 25 cultivated varieties and 18 ornamental plantings of pampas grass. All plants were genotyped using 10 polymorphic molecular markers (microsatellites).

Results indicate high levels of differentiation among pampas grass populations, consistent with colonization of new areas with a small number of colonists. Based on microsatellite markers, cultivated varieties clustered into cultivars clonally propagated by nurseries, varieties 'Pink' and 'White' from mail-order and online vendors propagated by seed, ornamental plantings, and unnamed generic pampas grass sold by nurseries.

The majority of wild populations appear to be derived from ornamental plantings and unnamed pampas grass sold by nurseries. In contrast, all jubata grass populations in California appear to consist of a single clone. This single clonal genotype is identical to that of 28 jubata grass samples obtained from Maui, Hawaii, indicating the same clone is invasive in both California and Hawaii. Initial evaluations of population sex ratios in pampas grass populations indicate plants can be female, male, or hemaphrodite. Further, we observe continuous morphological variation between maleness and hermaphroditism with varying degrees of female functionality/nonfunctionality.

First-year
progress

Plants and/or leaves were collected from cultivars and ornamental plantings of pampas grass, and wild populations of pampas grass and jubata grass, throughout California during the late summer and fall of 2003. Molecular genetic studies of the collected plant material will allow us to trace the spread of pampas grass and jubata grass and determine the sources of invasive plants. Early results, using only a few molecular markers, and pampas grass from five locations and jubata grass from three locations, indicate that pampas grass is genetically diverse in contrast to jubata grass, which lacks genetic variability. Preliminary results also suggest that the sources of pampas grass invasions in northern and southern California were different cultivars although these early results need to be confirmed by additional studies.

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