Spatial Structure and Genetic Variation of a Mangrove Species (Avicennia Marina (Forssk.) Vierh) in the Farasan Archipelago
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Article Spatial Structure and Genetic Variation of a Mangrove Species (Avicennia marina (Forssk.) Vierh) in the Farasan Archipelago Rahmah N. Al-Qthanin 1,* and Samah A. Alharbi 2 1 Biology Department, College of Sciences, King Khalid University, Abha 61421, Saudi Arabia 2 Biology Department, College of Applied Sciences, Umm-Al-Qura University, Makkah 21421, Saudi Arabia; [email protected] * Correspondence: [email protected] Received: 12 August 2020; Accepted: 18 November 2020; Published: 30 November 2020 Abstract: Avicennia marina (Forssk.) Vierh is distributed in patches along the Farasan archipelago coast and is the most common mangrove species in the Red Sea. However, to date, no studies have been directed towards understanding its genetic variation in the Farasan archipelago. In this investigation, genetic variations within and among natural populations of Avicennia marina in the Farasan archipelago were studied using 15 microsatellite markers. The study found 142 alleles on 15 loci in nine populations. The observed (Ho) and expected (He) heterozygosity values were 0.351 and 0.391, respectively, which are much lower than those of earlier studies on A. marina in the Arabian Gulf. An inbreeding effect from self-pollination might explain its heterozygote deficiency. Population genetic differentiation (FST = 0.301) was similar to other mangrove species. Our findings suggest that the sea current direction and coastal geomorphology might affect genetic dispersal of A. marina. The more isolated populations with fewer connections by sea currents exhibited lower genetic variation and differentiation between populations. The genetic clustering of populations fell into three main groups—Group 1 (populations of Farasan Alkabir Island), Group 2 (populations of Sajid Island), and Group 3 (mix of one population of Farasan Alkabir Island and a population of Zifaf Island). More genetic variation and less genetic differentiation occurred when the population was not isolated and had a direct connection with sea currents. Both of these factors contributed to limited propagule dispersal and produced significant structures among the population. It is expected that the results of this research will be useful in determining policy and species-conservation strategies and in the rehabilitation of A. marina mangrove stands on the Farasan islands in an effort to save this significant natural resource. Keywords: Avicennia marina; genetic variation; conservation; the Farasan archipelago; genetic structure 1. Introduction Mangroves are trees or shrubs found in coastal areas, lagoons, estuaries, and deltas. They form the main vegetation in tidal and saline wetlands, in turn providing habitats for many other species [1]. They are present in the tropics [2], and grow in mud with a flow of freshwater bringing nutrients [3]. The most-extensive mangrove areas are situated in Asia (42%), Africa (20%), North and Central America (15%), Oceania (12%), and finally South America (11%) [4]. They play a vital role in marine life and fisheries by providing food and shelter for a large and varied group of marine organisms, including fish and shellfish [5], and protect coastal areas from storms and sea level rises [6]. Globally, many mangrove forests have been converted to productive lands for agriculture and aquaculture [7]. Estimates of loss range from 35 to 86% [4,8] for the last two decades. Approximately only 6.9% of Forests 2020, 11, 1287; doi:10.3390/f11121287 www.mdpi.com/journal/forests ForestsForests 20202020,, 1111,, 1287x FOR PEER REVIEW 22 of 1819 Estimates of loss range from 35 to 86% [4,8] for the last two decades. Approximately only 6.9% of the theworld’s world’s mangrove mangrove areas areas are are protected protected under under the the International International Union Union for for Conservation Conservation of NatureNature (IUCN)(IUCN) programprogram [[8].8]. TheseThese mangrovemangrove communities are vulnerable to threats,threats, mainlymainly duedue toto humanhuman impactimpact throughthrough coastalcoastal construction,construction, industrialindustrial pollution, littering, a loss of water quality, andand thethe developmentdevelopment of fisheriesfisheries [[9–12].9–12]. In addition, in some areas, naturalnatural disasters,disasters, such asas earthquakes,earthquakes, tsunamis,tsunamis, coastalcoastal erosionerosion [[13,14],13,14], andand climateclimate changechange [[15,16],15,16], threatenthreaten mangroves.mangroves. TheThe firstfirst recordrecord ofof mangrovesmangroves inin thethe RedRed SeaSea datesdates toto 323323 BCBC [17[17,18].,18]. MangrovesMangroves are notnot continuouslycontinuously distributeddistributed in thein the Red SeaRed andSea show and evidenceshow evidence of being atof the being edge ofat theirthe environmentaledge of their range,environmental being stunted range, in being comparison stunted to in other comparison deltaic and to estuaryother deltaic areas inand the estuary world [areas19]. This in the is mainly world due[19]. toThis ahigh is mainly salinity, due reaching to a high aroundsalinity, 40%, reaching as well around as poor 40%, soil as well textures as poor and soil very textures high seawaterand very temperatureshigh seawater (32 temperatures◦C) [19,20]. A(32 study °C) [19,20]. conducted A stud by Saifullahy conducted (1997) by revealedSaifullah that (1997) the revealed conditions that in the southernconditions part in the of thesouthern Red Sea part are of more the Red favorable Sea are for more mangroves favorable than for themangroves northern than parts the because northern of higherparts because nutrient of concentrations, higher nutrient more concentrations, rainfall, many more runnels, rainfall, and many a lower runnels, salinity and due a lower to the salinity connection due toto thethe Indianconnection ocean to andthe Indian the water ocean flow and from the thewater Gulf flow of Adenfrom the into Gulf the of Red Aden Sea into [21]. the Although Red Sea large [21]. standsAlthough of mangroves large stands are of found mangroves along the are Red found Sea mainlandalong the coast, Red theSea Farasanmainland archipelago, coast, the whichFarasan is anarchipelago, uplifted fossil which coral is an reef uplifted in the southernfossil coral part reef of in the the Arabian southern coast part of of the the Red Arabian Sea [ 22coast–24 ],of only the Red has 2 aroundSea [22–24], 36.15 only km hasof mangrovesaround 36.15 [25 km²]. The of mangroves Farasan archipelago [25]. The Farasan is characterized archipelago by a is high characterized humidity, highby a meanhigh humidity, annual temperatures, high mean annual and low temperatures, rainfall [26]. Twoand low species rainfall of mangrove [26]. Two occur species on theseof mangrove islands: Avicenniaoccur on these marina islands:and the Avicennia less common marinaRhizophora and the less mucronata common[27 Rhizophora,28]. mucronata [27,28]. ThisThis paperpaper focusesfocuses on onAvicennia Avicennia marina marina(Avicenniaceae) (Avicenniaceae) in thein the Farasan Farasa archipelagon archipelago because because it is it the is dominantthe dominant mangrove mangrove species species and becauseand because it can it reproduce can reproduce and thrive and acrossthrive aacross wide rangea wide of range climatic, of saline,climatic, and saline, tidal and conditions tidal conditions [29,30]. [29,30]. It has several It has several mechanisms mechanisms leading leading to a high to a tolerance high tolerance of high of salinityhigh salinity levels, levels, reaching reachi 70%ng for 70% populations for populations of Avicennia of Avicennia marina in marina the Arabian in the GulfArabian [31]. Gulf The scientific[31]. The literaturescientific literature on A. marina on A.found marina on found the Farasan on the Farasan islands islands is limited is limited in coverage in coverage of the of distribution the distribution and coastaland coastal ecology. ecology. The The distribution distribution size size of ofA. A. marina marinain in the the archipelago archipelago ranges ranges from from stuntedstunted bushes, usuallyusually growinggrowing on on the the inner inner fringes fringes of theof the stand, stand, to well-developed to well-developed trees trees reaching reaching up to 4up m to in 4 height. m in Thisheight. species This species has been has reported been reported on three on major three islands,major islands, including including Farasan Farasan Al-Kabir, Al-Kabir, Sajid, Sajid, and Zifaf and (FigureZifaf (Figure1); the 1); largest the largest stands stands occur withinoccur within the Port the of Port Farasan of Farasan Alkabir Alkabir Island Island [32,33]. [32,33]. FigureFigure 1. GeographicalGeographical locations locations of the of thenine ninenatural natural populations populations of Avicennia of Avicennia marina marinain the Farasanin the Farasanarchipelago. archipelago. A.A. marinamarinais is important important for for the the people people of of the the islands islands and and there there is ais strategy a strategy for for its conservationits conservation in the in naturalthe natural environment environment [11]. [11]. However, Howeve ther, loss the and loss fragmentation and fragmentation of A. marina of A. aremarina enormous are enormous at a regional at a level.regional Camel level. grazing Camel hasgrazing been has reported been reported to be a major to be problema major problem causing causing the degradation the degradation of A. marina of A., excludingmarina, excluding only a few only stands a few growing stands ongrowing Zifaf Island on Zifaf