Proceedings of the Third International Conference on Invasive ...

Chapter 1: Spartina Biology<strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> <strong>Third</strong> <strong>International</strong> <strong>Conference</strong> on Invasive Spartinafollowed by alloploid speciation (S. anglica), involvingrelated, but more divergent parental species (S. alternifloraand S. maritima) on <strong>the</strong> o<strong>the</strong>r hand.In California, S. alterniflora was deliberately introducedin <strong>the</strong> mid-1970s in San Francisco Bay where it now cooccurswith <strong>the</strong> native S. foliosa (Daehler and Strong 1997).D. Strong and his co-workers have extensively studied <strong>the</strong>ecological and evolutionary consequences <strong>of</strong> thisintroduction (Ayres and Strong 2010). Hybridizationbetween <strong>the</strong>se two outcrossing, wind-pollinated speciesoccurs during <strong>the</strong> overlap <strong>of</strong> <strong>the</strong>ir flowering periods and hasbeen shown to occur bi-directionally (Antilla et al. 2000).Recurrent backcrosses have resulted in hybrid swarms thatdisplay most frequently <strong>the</strong> chloroplast haplotype <strong>of</strong> S.foliosa and up to 90% <strong>of</strong> <strong>the</strong> nuclear markers specific to S.alterniflora (Ayres et al. 1999; Antilla et al. 2000). Thesehybrids are rapidly spreading, and <strong>the</strong>y are now consideredas a conservation threat to <strong>the</strong> native S. foliosa populations.Reticulate events recently recorded in California alsoinvolve S. densiflora that has been introduced from Chile:Baumel et al. (2002a) reported an unexpected phylogeneticincongruence between different molecular data sets for <strong>the</strong>phylogenetic placement <strong>of</strong> a S. densiflora sample fromHumboldt Bay (California), and have consequentlyinterpreted this incongruence as possibly resulting fromhybridization with S. foliosa or S. alterniflora. Although <strong>the</strong>history <strong>of</strong> S. densiflora in both its native and introducedrange needs fur<strong>the</strong>r molecular investigation, <strong>the</strong> recentdiscovery <strong>of</strong> hybrids between S. densiflora and S. foliosa in<strong>the</strong> San Francisco Bay (Ayres and Lee 2010) confirms thathybridization may take place even between distantly relatedSpartina species.In western Europe, S. alterniflora has been accidentallyintroduced by shipping ballast at <strong>the</strong> end <strong>of</strong> <strong>the</strong> 19 th century.In Southampton Bay (England) hybridization with S.maritima resulted in a first generation hybrid S. townsendii,that is still growing vegetatively near Hy<strong>the</strong>. Chromosomedoubling gave rise to a new fertile allopolyploid species, S.anglica that has rapidly expanded in range (Gray andRaybould 1997). Spartina anglica and S. x townsendii have35403S. foliosa1S. alternifloraS. maritimaBidirectional introgressionSterile F1 hybrids&Allopolyploid speciationS. foliosaS. alternifloraS. maritimaFig. 1. Phylogenetic relationships <strong>of</strong> three Spartina species involved in recent hybridizations. Branch lengths are proportional to <strong>the</strong> number <strong>of</strong> nucleotidechanges (above <strong>the</strong> branches) recorded from <strong>the</strong> analysis <strong>of</strong> two nuclear (ITS and Waxy) and one chloroplast (trnT-trnL spacer) DNA sequences(data from Baumel et al. 2002a). The map displays <strong>the</strong> natural range <strong>of</strong> <strong>the</strong>se species and <strong>the</strong> two arrows indicate <strong>the</strong> recent introductions <strong>of</strong> Spartinaalterniflora.-16-