Tuesday, 27 March 2012

Marine species threatened through excessive hybridization


Marine species threatened through excessive hybridization

Isolated marine species that are capable of hybridizing with migratory relatives are found to be at a higher risk of being bred out of existence. This concept is highlighted particularly well in the study of hybridization of the migrating species the Yellowfin Bream (Acanthopagrus australis) and the geographically isolated species the Black Bream (Acanthopagrus butcheri) by D.G Roberts, C. A. Gray, R.J West and D.J Ayre(2010) .  Populations in which these species interact, particularly populations in permanently closed estuarine systems have been surveyed using microsatellite and mitochondrial DNA (mtDNA) markers to determine the presence and degree of hybridization within geographically isolated populations.

  Figure 1. A.butcheri, distinguished by the length of its pectoral fin and pointy head.
Figure 2. A.australis, as the common name suggests the fish has a yellow anal fin and a more rounded head then that of the Black bream

 As  A.butcheri is an obligately estuarine species there is often noticeable genetic diversity between different populations within the species (Chaplin et al. 1998) which is an influential factor on the population’s ability to breed with A.australis. Although A.butcheri exhibits variation between populations due to genetic divergence through geographical isolation it was not significant in the area the study was taken place (NSW far south coast) in comparison to that of the larger estuaries in Victoria (Farrington et al. 2000). Through this study it has become apparent that A.butcheri  (estuarine obligate) is at threat of losing its genetic identity due to hybridization with A.australis contained within the same environment. The study used a diversity of ages amongst the fish to help determine if the rate of hybridization is increasing and how the genetics differ between juvenile hybrids and adults in terms of alleles per locus. This is how the species differ most greatly with A.australis possessing approximately twice as many alleles per locus (0.815 ± 0.037 vs. 0.610 ± 0.084) compared to that of the A.butcheri (Roberts et al. 2009). In reference to this data the study used a qi- value which represents the amount of A.butcheri ancestry present in a specimen; with this a hypothesis can be made as to the continuation of the species, A.butcheri. The study found that out of the 688 fish tested 95% were either hybrid (309) or A.australis(347) leaving a scarce population of A.butcheri (32)(Roberts et al. 2009). This furthermore highlights that with high levels of introgression the hybridizing species is under a serious threat of being bred out of the population and potentially leading this species to extinction (Levin et al. 1996; Rhymer & Simberloff (1996)).

Table 1 Proportion of juvenile Acanthopagrus australis (Aa), Acanthopagrus butcheri (Ab), and hybrids (Hyb) in five southeastern Australian lagoons (including temporal replication in two lagoons, Coila)*
                                                                                    

q-Threshold
q-Threshold
q-Threshold
Lagoon, year
(sample size, n)
0.05
0.10
0.20
Aa
Ab
   Hyb
Aa
Ab
   Hyb
Aa
Ab
   Hyb
Coila, 1999 (89)†
0
0.12
 0.88
 0
 0.20
 0.80
0
 0.47
 0.53
Coila, 2001 (81)
0
0.09
 0.91
0
 0.15
 0.85
0
 0.46
 0.54
Coila, 2003 (79)
0    
   0.03
0.97
0
 0.13
 0.87
0
 0.34
 0.65
Tuross, 2001 (179)
0.85 
0.01
0.14
0.91
 0.01
 0.08
0.95
0.02
0.03
Brou, 1995 (19)
0
0.21
0.79
0
0.26
 0.74
 0
 0.42
0.58
Brou, 1999 (38)
0
0.21
0.79
0
0.34
 0.63
0
 0.5
0.5
Brou, 2001 (170)
0.01
0.07
0.92
0.01
0.22
0.78
0.01
0.52
0.48
Brou, 2003 (57)
0
0.19
0.81
0
0.28
0.72
0
0.39
0.61

Corunna, 2001 (147)
0.66
0.07
0.27
0.73
0.08
0.19
0.74
0.14
0.12
Wallaga, 2001 (117)
0.87
0.01
0.12
0.92
0.01
0.07
0.96
0.01
0.03
Overall, 2001 (688)
0.50
0.05
0.45
0.55
0.09
0.36
0.56
0.22
0.22
*The specific-species status of each individual was determined using admixture analysis based on eight microsatellite loci. The effect of varying the q-threshold value (qi = the average proportion of A. butcheri genome) used to distinguish parentals and hybrids is also displayed.
†All 2001 fish were caught as juveniles, just after spawning in A. butcheri, whereas all other fish were caught as adults, with the age of each fish estimated from its otoliths. Based on the age and date of capture, individual fish were separated into specific cohorts, i.e. year of birth—1995, 1999 and 2003.(Roberts et al. 2009)
                                                                 
Although this study is specific to the two species A.australis and A.butcheri it assists in raising the issue of other possible hybridization swamping as well as extinctions. This has been before seen in situations worldwide where introduced species have hybridized with a local population and eventually resulting in genomic extinction of both rare and geographically isolated species (Perry et al. 2001). With this in mind other species may be vulnerable and without the use of genetics (microsatellite, mitochondrial DNA markers and polymerase chain reaction amplification [PCR]) we would not be capable of identifying what could result in the extinction of a genome or species.




 Reference list
Chaplin JA, Baudains GA, Gills HS, Mc Culloch R, Potter IC (1998) States that populations of A.butcheri in different estuaries are genetically distinct, International journal of salt lake research, 6, 301-321.
David G. Roberts, Charles A. Gray, Ronald J. West and David J. Ayre (2009) Marine Genetic Swamping: hybrids replace an obligately estuarine fish Available at:   http://www.ncbi.nlm.nih.gov/pubmed/20070520
Farrington LW, Austin CM, Coutin PC (2000) Allozyme variation and stock structure in the Black bream, Acanthopagrus butcheri  (Munro) (Sparidae) in southern Australia: implications for fisheries management, aquaculture and taxonomic relationship with Acanthopagrus australis (Gunther). Fisheries Management and Ecology, 7, 265-279. Avaliable at: http://www.dpi.nsw.gov.au/__data/assets/pdf_file/0006/344661/WF_2008_Output-1727_Gray_RFSTEC-report-re-Biology-of-Black-Bream_REPORT.pdf
Levin DA, FranciscoOrtega J, Jansen RK (1996) Hybridization and the extinction of rare plant species. Conservation Biology, 10, 10-16
Perry WL, Feder JL, Lodge DM (2001b) Researched hybridization between migratory and residential crayfish. Conservation Biology, 15, 1656-1666.
Rhymer JM, Simberloff D (1996) How hybridization can lead to extinction. Annual Review of Ecology and Systematics, 27, 83–109
Note: figures 1 & 2 were taken by me!

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