The use of monosex fish is intrinsically desirable in a variety of fish species in a range of aquaculture production systems. The potential advantages sought from their use may include one or more of the following features: achievement of higher average growth rate, elimination of reproduction, reduction of sexual/territorial behaviour, reduction of variation in harvest size, and reduction of risk of environmental impact resulting from escapes of exotic species.Fish as a group have systems of sex determination which are of considerable biological interest and significance for studies in evolutionary biology. However, they are very variable, relatively poorly understood and give rise to much variation in sex ratio between, and within, species. Enough is known, however, to enable us to say that these systems are often employed in ways which sharply distinguish the fishes from groups such as mammals, birds and reptiles. As a consequence, manipulations of sexual phenotype designed to produce monosex populations are not straightforward and the results are not necessarily predictable. This paper reviews the techniques for production of monosex males, and considers in detail the case of the Y Y / G M T technology in the Nile tilapia, which is the only example of a genetic technology for the production of monosex males so far widely adopted by the aquaculture industry. The considerable benefits accruing from the use of GMT are described. An attempt at projecting future developments in this area of aquaculture is made. 0 0 4 4 -8 4 8 6 / 0 1 / $ -see front matter ©2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 4 -8 4 8 6 ( 0 1 ) 0 0 5 9 0 -7 284 JA. Beardmore et al / Aquaculture 197 (2001) 283-301
StLircIi gel electrophoi-esis was used to study \-ariation at I I loci in mussels sampled mainly from British co'istal sites. Two types of mussel were identified, 2\f>sti"u ediilis, tlie common mussel rind its soutliern relatit e .\fvti/iir ga//n/~~,/,,or~iricin/i~. Se\ era1 partially dingnortic loci were used to miip tlie distrihtition of tlie t\vo forms. .\fvti/riJ ediilis \\as present ;it all sites sampled in Britain and Ireland hut \\as at low frequency in S\V England; .\I. ga//ul,ioi'incia/ir \vas detected in S\V England, tlie south and west of Ireland. Scotland and S E England. hut was absent from soutli \Vales. the Irish sea co;ists of tValt-s and Ireland. and SE England. Ap;irt fi-om tlie occurrence of . i f . gn//upinz,inrin/is in NE England. this distribution conforms bvith tlie results of studies using morphological characters and parallels the distrihution of many other soutlicrn species in Britain. At thc microgeographical l e d , .If. c d i i / i~ was foriitd to prcfrr more slieltered a n d rstiiarine conditions than :\I. ,~a//opmr,iiiria/i~. .Analysis rising the hest diagnostic loci sliowed that tiybridizati(in is occurring between .if, rdiilis and .\I. gn/lopiur~in~-ia/ir at all localities where the) occiir sympatrically hut that tlie extent of hybridization \aries considerahl) hetween localities. 'The distribution of localities ha\ ing Iiigli proportions of hybrid individuals is hest interpreted hy assuming that 11) hrids ha\ e higher fitness than parental tyries at these localities. :\ stlid! \v.is made o f \ ariation within and between those localities where only .\f. ~diilir indi\ idualsTvei-e ohser\.ed. Little significant geographic \.ariation in dlele frequency was detected, hut significcrnt deficits of 1ieteroz)gotes compared with H;irdy-lVeinberg expectations n e r e seen for most loci. .\n:ilysis suggests that the \Valilund effect is not involxd and that the most likely cause of tlie deficit is low frequencies (if niill 'illeles. In .\I. tdii/iJ no differences in phenotypic \.ariance in shell height and width v w e ohserved hetween samples of multiply lietcrozygoiis and multiply liomozygoiis individuals ;ind no qriietic differences were found bet\vern juveniles and adults. Overall little evidence was found t h a t Iialnncing selectinn is responsible for m'rintrn;ince of the polymoi-phisms studied in .if. edulir. 1 lie ~iatterii ofgcogr;ipliic \.ariation in gene frequencies in d i f v t i /~i s in the British Isles is discussed in relation to variation in tlie south and north of F.rirope and S o r t h . h e r i c a . It is concluded that steep clines in gene li-eqiiencies in .\I. idrrlis ohser\-ed h! otlier \viirkers in tlie Bnltir and in Long 1sl;ind Sound cannot tie .ittribiited to the presencr of .\I. gn//ol"ui,incin/i.
Abstract. Zoanthid taxonomy is currently in a state of chaos, with many described species very few of which can be reliably identified. As part of a genetically based, objec-
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