Gene flow in carrots

Authors: MANDEL, JENNIFER - University Of Memphis; Brunet, Johanne

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 11/17/2018
Publication Date: 5/14/2019
Citation: Mandel, J.R., Brunet, J. 2019. Gene flow in carrot. In: P. W. Simon, M. Iorizzo, D. Grzebelus and R. Baranski, editors. The Carrot Genome, Compendium of Plant Genomes. Springer Nature: Cham, Switzerland. p. 59-76. https://doi.org/10.1007/978-3-030-03389-7_4.
DOI: https://doi.org/10.1007/978-3-030-03389-7_4

Interpretive Summary: Carrot is an important crop and maintaining cultivar purity is a major concern for the carrot industry. Movement of genes among carrot fields can affect cultivar purity. Therefore, isolation distances have been set and a minimum of 1000 m is recommended between cultivar fields and distances of 3-5 km are often maintained among different root color types. But cultivar purity can also be affected by gene flow from wild carrot to cultivated fields. Cultivated and wild carrot belong to the same species, are interfertile, often grow in close proximity so gene flow from wild to cultivated carrot is a great concern. Wild carrot introduced into cultivated carrot fields grown for roots are often expressed as early bolters and these can affect cultivar purity and carrot quality. Fields of seed production in carrots are grown away from wild carrots and when present wild carrots are mowed to prevent their reproduction and introduction of wild carrot genes into cultivated carrots. Wild carrots have white bitter roots as opposed to the more common orange or purple cultivated carrots. Besides gene flow from wild to cultivated carrot, cultivar genes may also move into wild carrot populations. Because wild carrots tend to be weedy and have been declared invasives in many states in the USA, concerns of increased weediness resulting from introduction of cultivar genes is real. It is therefore important to examine gene flow from cultivated carrot to wild carrot. Each successful gene flow event will produce a seed that will grow into a first generation hybrid between wild and cultivated carrot. These F1 hybrids can then mate with wild carrot to produce first and then later-generation backcrosses. As these matings occur in a wild carrot population the frequency of the cultivar genes can increase. This is the process of spread within a population. But cultivar genes can also move to other wild carrot populations and it is thus important to measure gene flow among wild carrot populations to obtain a better picture of the process of introgression over the wild carrot landscape. Wild carrot, although thought as biennial, can exhibit a variety of life history strategies, present in different proportions in wild carrot populations. These life history strategies can differentially affect the growth rate of populations and therefore the range expansion of wild carrot. In this chapter, we first discuss the characteristics of carrot that facilitate gene flow before reviewing the studies of gene flow that used molecular markers to quantify gene flow between crop fields, between crop and wild, and among wild carrot populations. After summarizing the evidence from gene flow studies, we discuss the consequences of these different types of gene flow (among cultivars, between crop and wild and among wild). An important consequence of gene flow between cultivated and wild relatives is whether the cultivar genes or genetically modified gene from a crop spread in the wild populations, a process called introgression. It is a major goal of biotechnology risk assessment to improve predictions of the fate of these escaped genes. We discuss the process of introgression and suggest as a priority for future studies to incorporate population dynamics with population genetics when modeling the fate of introduced genes. A better understanding of the factors that influence introgression of escaped genes will facilitate the design of management strategies to better contain and limit their spread. This information will benefit, carrot farmers and the carrot industry together with scientists and people concerned

Technical Abstract: There are many reasons to study gene flow in carrot and many important questions to be answered. Carrot is an important crop and maintaining the purity of cultivars is a priority for the carrot industry. Gene flow among cultivars is reduced to maintain cultivar purity by setting minimum isolation distances among fields. But gene flow from wild to cultivated carrot can also affect cultivar purity and carrot quality. Cultivated and wild carrot belong to the same species, are interfertile, often grow in close proximity such that gene flow from wild to cultivated carrot is a great concern. Such gene flow is often noted by the presence of early bolters in carrot fields. But gene flow can also occur from cultivated to wild carrot. Given wild carrots are weedy and have been declared invasives in a number of states in the USA, the escape of cultivar genes and their spread into wild populations is a question of interest as there is a chance that escaped genes could render wild carrot even more invasive. The process of introgression of cultivar genes in wild carrot populations includes the spread of the gene within and among wild carrot populations. Therefore, to understand the process of introgression in wild carrots it is important to obtain estimates of gene flow among wild carrot populations. In this study, we first present characteristics of carrots that will affect gene flow and discuss dispersal via pollen by insect pollinators and via seeds by wind and animals. Although it is often referred to as a biennial, we introduce the various life history strategies observed in wild carrot populations as these can impact population growth and the range expansion of wild carrots over the landscape. We then review the studies of gene flow between crops, between crop and wild carrot and among wild carrot populations, concentrating on studies that used molecular markers. The consequences of these different types of gene flow (among cultivars, between crop and wild and among wild) are then discussed. A major goal of biotechnology risk assessment is to improve predictions of the fate of escaped genes. We suggest as a priority for future studies to incorporate population dynamics with population genetics when modeling the fate of introduced genes. Improving our understanding of the factors that affect the spread of escaped genes will lead to the design of better management strategies to contain and limit their spread.