Abstract Detail


Fishbein, Mark [1], Wagner, Carl [2], Andreev, Victor [3], Straub, Shannon [4].

What is the longleaf milkweed of the western Gulf Coast?

Asclepias longifolia (longleaf milkweed) and A. hirtella (green milkweed, prairie milkweed) have most commonly been treated as species, but also as conspecific subspecies. Distinct among all other North American milkweeds by virtue of hemispherical to spherical inflorescences of small, long-pedicellate, comet-like flowers, distinctions between the species blur where their ranges approach. Asclepias longifolia is found on the Atlantic and Gulf coastal plains and peninsular Florida. Asclepias hirtella is widespread across the tallgrass prairie region, ranging into glades and forest openings in the Ozarks, Cumberland Plateau, and adjacent regions. The taxa have been distinguished by a combination of traits: A. hirtella is taller, has more numerous umbels consisting of a greater number of flowers, and has hirtellous pedicels; whereas is A. longifolia is shorter, has fewer umbels with fewer flowers, and appressed, curved-puberulent pedicels. However, plants occurring from the fall line in Georgia west to eastern Texas exhibit intermediate morphologies and have been variously ascribed to each species. In particular, plants of the Gulf coastal plain west of the Mississippi River have almost invariably been treated as A. longifolia, despite having hirtellous pedicels. We hypothesized that all these intermediate populations represent short-statured A. hirtella, and that the two taxa represent distinct species showing evidence of past introgression. We collected Hyb-Seq data, consisting of 3300 nuclear loci and plastid genomes, from range-wide samples of both species, including the geographic zone of intermediacy. The phylogeny of plastid genomes showed reciprocal monophyly of the species as circumscribed according to our hyphothesis (i.e., western Gulf populations belong to the A. hirtella clade). Single-nucleotide polymorphism data obtained from the nuclear loci also show strong differentiation between the species consistent with our hypothesis and low levels of admixture. We conclude that the variation in A. hirtella resulting in A. longifolia­-like plants is due in part to environmental factors and in part to past introgression.

1 - Oklahoma State University, Dept Of Plant Biology, Ecology & Evolution, 301 Physical Science, Stillwater, OK, 74078, United States
2 - Hobart & William Smith Colleges, Geneva, NY, USA
3 - Oklahoma State University, 301 Physical Sciences, 301 Physical Sciences, Stillwater, OK, 74078, United States
4 - Hobart And William Smith Colleges, Department Of Biology, 300 Pulteney St., Geneva, NY, 14456, United States

species delimitation
targeted sequencing
nuclear genes
Taxonomy .

Presentation Type: Oral Paper
Number: SYST III001
Abstract ID:548
Candidate for Awards:None

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