Hybrid Bridges to Gene Flow: A Case Study in Milkweeds (Asclepias)

Abstract

Abstract. Natural hybridization occurs throughout areas of sympatry for the North American milkweeds Asclepias exaltata and A. syriaca (Asclepiadaceae), even though the formation of F 1 hybrid seed is a rare event. For introgressive hybridization to proceed, F 1 and advanced hybrids must be released from reproductive barriers and successfully mate with one or both parental species. I investigated the mating system of natural hybrids between A. exaltata and A. syriaca in three populations in Shenandoah National Park, Virginia. Allozyme data and a maximum-likelihood procedure were used to estimate the frequency of six genotypic classes (parentals, F 1 , F 2 , and backcrosses) of the hybridizing populations, the pollinia received by hybrid plants, and the paternal parents of seeds produced by hybrids. F 1 hybrids, backcross A. syriaca, and parental A. syriaca individuals were common in three hybrid populations. Even though self-pollinations and interhybrid pollinations were common, F 2 seed production and the occurrence of F 2 individuals were rare in hybrid populations. Hybrid plants received more pollen from A. syriaca than A. exaltata, which resulted in the production of more backcross-A. syriaca seed than backcross-A. exaltata seed. Asclepias exaltata was rare in the hybrid populations, but A. exaltata pollinia were received by hybrids and this species sired between 15% and 36% of the seeds produced on hybrids. The potential for introgression with A. exaltata populations is lower because this species is unsuccessful as the maternal parent in interspecific and backcross hand-pollinations. The asymetry of hybridization with A. syriaca as the maternal parent is further supported by the incorporation of maternally inherited chloroplast DNA markers in hybrids. Hybrid milkweeds frequently backcross with both parental species and may be released from the reproductive barriers that limit the formation of F 1 hybrids in natural populations. The direction of interspecific gene flow and introgression in milkweeds is influenced by the reproductive biology of hybrids, the constituency of the surrounding population, and failure of some crosses to produce seeds. Finally, introgressive hybridization remains an important evolutionary force even when the initial formation of F 1 hybrids in natural populations is rare.

Interspecific hybridization in the genus Hieracium s. str.: evidence for bidirectional gene flow and spontaneous allopolyploidization

“…Pollen movement might be in fact more efficient than we previously thought, and many of the hybrid seeds probably drop to the ground near the maternal parents without long dispersal. A similar pattern has been observed in the genus Asclepias, which is also adapted for long distance seed dispersal (Broyles 2002). Furthermore, we cannot exclude a potential advantageous effect the maternal parent might have on the hybrid's adaptation to the same habitat.…”

Post-pollination mechanisms in Nicotiana longiflora and N. plumbaginifolia: pollen tube growth rate, offspring paternity and hybridization

“…Because in natural populations pollinators have a strong preference for N. longiflora over N. plumbaginifolia and because they may visit hybrids and parental species indiscriminately (Wendt et al 2001;Ippolito et al 2004); asymmetric hybridization and introgression between these species may be occurring, such that more N. plumbaginifolia genes are introduced into N. longiflora than in the reverse direction; such as has been documented for other species (Broyles 2002;Martin and Willis 2007). Therefore, further studies should address how much introgression has occurred and how much of the original genome from each species is still conserved.…”

Differential pollinator effectiveness and importance in a milkweed (Asclepias, Apocynaceae) hybrid zone

“…March 2012] STOEPLER ET AL. -POLLINATOR EFFECTIVENESS IN A MILKWEED HYBRID ZONE rare ( Broyles et al, 1996 ;Broyles, 2002 ). Generalist hymenopteran and lepidopteran pollinators of these species at SNP ( Broyles et al, 1996 ) are typical of these and other common and widespread milkweeds ( Robertson, 1891( Robertson, , 1928Betz et al, 1994 ;Wyatt and Broyles, 1994 ;Fishbein and Venable, 1996a ;Theiss et al, 2007 ).…”