February 26, 2015
Fitness and Morphological Variation in Native, Non-Native, and Hybrid Echinacea Seedlings
Jill Pastick '14 graduate of Biology.
Jillian Pastick
Department of Biology
Lake Forest College
Lake Forest, Illinois 60045
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Introduction
Hybridization between native and non-native plant species is developing into a topic of interest as the introduction of non-native species becomes more common. Hybridization is a process seen frequently in nature, occurring in at least 25% of all plant species (Rhymer and Simberloff 1996, Ellstrand et al. 2013). The occurrence of hybridization between native species, non-native species, or native and non-native can have adverse effects on plant populations, including changes in morphological and phenotypic traits (Vilà et al. 2000). Hybridization between native and non-native species also has the potential to increase the genetic impact of non-native species on a community by creating new opportunities for introgression (gene exchange due to backcrossing of hybrids with parental plants), competition, or in some cases, extinction of native populations. For example, hybridization may result in an F1 population with faster growth and higher fitness, creating potential for the out-competition of wild plant populations (Arnold and Hodges 1995, Van Gaal 1998). Genetic changes and introgression in native species can also elicit differences in flowering, pollination, seed dispersal, seed set, and resistance to pathogens (Vilà et al. 2000). In some cases, these genetic, evolutionary, and morphological changes are detectable in a single generation (Neuhauser et al. 2003). However, in other cases, these changes are not detectable for multiple generations. Because the rates at which these changes occur are so variable, it can be useful to focus on the effects of species introduction and hybridization in a specific population over an extended period of time, such as, plant populations in a prairie habitat.
The North American Tallgrass Prairie, which once extended across 162 million hectares from Canada to Texas, has now been reduced to small patches located between agricultural fields and along roadways, retaining less than 1% of its original area (Sampson and Knopf 1994). Fragmentation in prairie patches, paired with the increased introduction of non-native species, has resulted in an array of ecological and genetic changes in prairie plant populations (Kluger et al. 2011, Ridley et al. 2011), making them appropriate locations for studying the impact of interactions between non-native and native plant species. The potential impacts of future introductions of non-native species into fragmented habitats can be better understood by examining the potential for hybridization and competition between a native and a non-native species in remnant prairie communities, where introduction have been known to occur.
Recently, a non-native species, E. pallida, was introduced into remnant tallgrass prairie plots in Minnesota and currently exists in conjunction with populations of a native species, E. angustifolia. E. angustifolia and E. pallida have been known to hybridize in nature (MacArthur 1968). Sanford-Long (2012) successfully crossed E. angustifolia and E. pallida via hand-pollination, demonstrating the likelihood of interspecific crosses between E. angustifolia and E. pallida. Likelihood of hybridization was based on compatibility rates of both the intraspecfic and interspecific crosses by observing the shriveling of styles and resulting seed set (Sanford-Long 2012: Unpublished). Observing how hybridization occurs in remnant prairie plots is crucial for gaining insight into current E. angustifolia population dynamics and for preventing unwanted introgression into remnant Minnesota prairie populations. It is still unclear whether or not there are repercussions of the introduction. As a result, focusing on the progeny of these hybrid crosses will provide a better understanding of the morphological and genetic effects of E. pallida’s presence in a prairie remnant.
I investigate the interspecific and intraspecfic interactions between two species of Echinacea co-existing in a remnant prairie in Kensington, Minnesota by comparing the morphology, emergence, and mortality of four crosses between E. angustifolia and E. pallida (AA, AP, PA, and PP, Table 1). I tested the hypothesis that the F1 generation from these crosses will result in the production of viable seed, which will exhibit variation in emergence and survival rates. I germinated 515 achenes obtained from these crosses, and measured the germination rates of progeny grown in a controlled environment. I then tested the hypothesis that leaf characteristics differ among the four cross types early in the seedlings’ growth. I took photographs of 7 day-old cotyledons in order to collect a series of measurements, including: cotyledon area, perimeter, length, width, and circularity (4π (area/perimeter2)). I also tracked seedling growth by measuring height and width of the first true leaf at three ages over a two-month period. Because Echinacea exhibits many of the same characteristics typical of a number of plants native to the tallgrass prairie, particularly those in the widespread family, Asteraceae (Wagenius and Lyons 2010), much of the information obtained from studying the interactions between E. pallida and E. angustifolia can be applied more broadly, to study hybridization and invasion of other plant communities in fragmented prairie habitats.
TABLE 1. The four crosses employed