Genetic Structure and Evolution of Species in the Mangrove Genus Avicennia (Avicenniaceae) in the Indo-West Pacific

Abstract

Allozyme variation in species of the mangrove genus Avicennia was screened in 25 populations collected from 22 locations in the Indo-West Pacific and eastern North America using 11 loci. Several fixed gene differences supported the specific status of Avicennia alba, A. integra, A. marina, and A. rumphiana from the Indo-West Pacific, and A. germinans from the Atlantic-East Pacific. The three varieties of A. marina, var. marina, var. eucalyptifolia, and var. australasica, had higher genetic similarities (Nei's l) and no fixed gene differences, confirming their conspecific status. Strong genetic structuring was observed in A. marina, with sharp changes in gene frequencies at the geographical margins of varietal distributions. The occurrence of alleles found otherwise in only one variety, in only immediately adjacent populations of another variety, provided evidence of introgession between varieties. The varieties appear to have diverged recently in the Pleistocene and are apparently not of ancient Cretaceous origin, as suggested earlier. Despite evidence of high degrees of outcrossing, gene flow among populations was relatively low (N,m :5 1-2), except where populations were geographically continuous, questioning assumptions that these widespread mangrove species achieve high levels of long-distance dispersal.

Evolutionary divergence in the pan‐Atlantic mangrove Avicennia germinans

“…However, this raises the question of why, if dispersal is so efficient, Avicennia is notably absent from apparently suitable sites that would not require trans-oceanic dispersal (Steinke, 1972). Recently, allozyme data from another widespread member of the genus, A. marina, have indicated relatively low levels of gene flow among populations, and a geographical structuring among varieties, with contiguous distributions along the Australian coastline (Duke et al, 1998). This would suggest that dispersal events are unlikely to have important effects on the genetic structure of geographically close populations, raising serious doubts that trans-oceanic dispersal could be an effective force preventing allopatric differentiation.…”

“…Sherrod & MacMillan (1985) suggested that differentiation of these groups of populations was owing to separate marine currents that have prevented east-west migration since recolonization of the Gulf of Mexico after the last glaciation. More recently, Duke et al (1998) have reported low levels of gene flow among fragmented populations of A. marina around the Australian coast, and have also questioned the capability of long-distance dispersal in Avicennia.…”

“…This event provides a date of isolation and an assumed beginning of divergence in this region that can be used in the comparison of levels of differentiation within the geographic range of A. germinans, and perhaps a reference for comparison with divergence in closely related species. Duke et al (1998) have reported divergence dates within A. marina around the Australian coast at 2 Million yr ago for var. australasica and 200 000 yr for vars marina and eucalyptifolia.…”

Connectivity of populations within and between major biogeographic regions of the tropical Pacific in Conus ebraeus, a widespread marine gastropod

“…These patterns are best illuminated through comparison of the distributions of genotypes of populations of widespread species. For example, even though a number of unrelated taxa show concordant genetic breaks between Indian and Pacific populations, presumably caused by historical isolation of the two basins during times of low sea level stands (Benzie and Stoddart 1992;Mcmillan and Palumbi 1995;Lavery et al 1996a;Miya and Nishida 1997;Benzie 1997, 1998;Duke et al 1998;Benzie 1999;Duda and Palumbi 1999b;Lessios et al 1999Lessios et al , 2001Williams et al 2002;Bay et al 2004;Teske et al 2005;Reid et al 2006;Crandall et al 2008), there is no equivalent concordance in breaks between the western and central Pacific areas of the Indo-West Pacific, or between localities within either of these areas. Vicariant events that could have influenced the genetic divergence between populations in the western and the central Pacific are not apparent in geologic history.…”

Genetic recolonization of mangrove: genetic diversity still increasing in the Mekong Delta 30 years after Agent Orange

“…Recent assessments have revised upwards the impact of Agent Orange on human health in Vietnam (Butler 2003); the results presented here suggest a similarly pervasive effects on the ecosystem. An encouraging result, however, is the suggested ability of Avicennia alba to recolonize from external sources despite low propagule dispersal in normal conditions (Duke et al 1998, Clarke & Kerrigan 2002; this is supported by strong genetic structure at the local scale in congeneric species (Giang et al 2003, Kado et al 2004, Arnaud-Haond et al 2006. This may indicate the existence of density-dependent migration success in this mangrove species, suggesting that estimates of population genetic structure or effective migration obtained in undisturbed conditions may not provide accurate predictions of recolonization potential after local extinction.…”