Genetic Differentiation in the Intertidal Zone in Populations of the Alga Enteromorpha Linza (Ulvales: Chlorophyta)

Marine Biology 97, 9-16 (1988) Marine

...... Biology | Springer-Verlag 1988

Genetic differentiation in the intertidal zone in populations of the alga Enteromorpha linza (Ulvales: Chlorophyta)

D.J. Innes *

Department of Ecology and Evolution, State University of New York; Stony Brook, New York 11794, USA

Abstract presence of genetic differentiation between high and low intertidal populations. Koehn etal. (1973) found signifi- Genetic differentiation within the intertidal zone was cant genotype frequency differences at an enzyme locus in examined in six populations of the asexually reproducing the mussel Modiolus demissus between samples from high alga Enterornorpha linza growing in the Long Island and low intertidal positions, but these differences were on- Sound, USA. Four of the five populations sampled in 1981 ly on the order of 3%. Weak associations between enzyme showed significant differentiation between high and low genotypes and intertidal position have also been found for intertidal positions with respect to the GOT-2 locus. The the mussel Mytilus californianus (Levinton and Fundiller, pattern of differentiation was consistent for samples col- 1975; Tracey et al., 1975) and the bivalve Macoma balthica lected at several times during the year with some seasonal (Green et al., 1983). A genetically determined shell-colour modifications. Four additional polymorphic loci, resolved polymorphism in Mytilus edulis (Innes and Haley, 1977; in 1982, identified a total of 13 five-locus genotypes or Newkirk, 1980) showed significant association with in- clones. Four of the six populations sampled in 1982 tertidal position (Mitton, 1977). Genetic differentiation was showed significant differences in clone frequency between found for the intertidal limpet Siphonariajeanea (Johnson high and low intertidal positions. Laboratory experiments and Black, 1982), but the patterns of differentiation were revealed differences in response to temperature among the inconsistent over space and time and were shown to be clones. At 24 ~ a high intertidal associated clone showed partially due to temporal variation in the numbers and an increase in growth, while low intertidal associated genotypes of recruits (Johnson and Black, 1984). In con- clones showed decreased growth compared to growth at trast, some studies have failed to document genetic differ- 15 ~ These results suggest that the microgeographic dif- entiation among individuals from different positions in the ferentiation observed for E. linza in the intertidal zone may intertidal zone (Flowerdew and Crisp, 1976; Black and in part be due to the differential adaptation of clones to Johnson, 1981). The absence of strong microgeographic different intertidal environments. Additional demographic differentiation within the intertidal zone for many inverte- information is needed for individual clones in order to de- brate species may be due to the continued recruitment of termine the role longevity, reproductive output, re- planktonic larvae originating from a mixture of intertidal cruitment and interclonal competition play in maintaining populations. In addition, temporal variation in the genetic the observed differentiation. composition of recruits may generate sporadic genetic differentiation between intertidal populations (Johnson and Black, 1982). Several species of algae have a broad distribution in the Introduction intertidal zone (Zaneveld, 1969; Femino and Mathieson, 1980; Gunnill, 1980; Mathieson etal., 1981; Oliger and Steep environmental gradients are found over short dis- Santelices, 1981). The upper limits among some species of tances in the intertidal zone of the coastal marine en- algae have been shown to be related to differential abilities vironment. Several studies have examined genetic vari- to tolerate high temperature and desiccation (Zaneveld, ation in intertidal invertebrate species to determine the 1969; Biebl, 1970; Mathieson and Burns, 1971; Lubcbenco, 1980; Oliger and Santelices, 1981) and the lower limits de- * Present address: Department of Biology, Memorial University termined by biological factors such as competition and her- St. John's, Newfoundland A1B 3X9, Canada bivory (Zaneveld, 1969; Chapman, 1973; Lubchenco, 10 D.J. Innes: Genetic differentiation in the intertidal zone

Q FP S -SH ATLANTIC # N OCEAN l 20 Km Fig. 1. Enteromorpha linza. Map of the six intertidal lo- NE calities sampled. Stony Brook (SB); Mill Pond (MP); Flax Pond (FP); Nissequogue East (NE); Nissequogue West (NW); Shinnecock (SH)

1980). However, there is a lack of information on whether 1984). In the Long Island Sound this species appears to re- the distribution of an algal species in the intertidal zone is produce only by asexual spores. The five-locus phenotypes accompanied by any genetic differentiation. have therefore been tentatively identified as 13 genetically Enteromorpha linza shows a wide vertical distribution distinct clones, with the understanding that additional in the intertidal zone along the shores of the Long Island polymorphic loci may split some of these clones into ad- Sound. In the Long Island Sound, E. linza reproduces by ditional clones (Innes and Yarish, 1984; Innes, 1987). free-floating asexual spores (Innes and Yarish, 1984) which In 1982 the SB and MP localities (Fig. 1) were selected would be expected to decrease genetic differentiation for a more extensive analysis of intertidal differentiation in among populations separated by small distances. However, Enteromorpha linza. On May 28, 1982 additional individ- a large amount of genetic differentiation was observed for uals were collected at the SB locality from two intermedi- populations of E. linza occurring in adjacent salinity habi- ate positions between the highest and lowest intertidal po- tats separated by only a few hundred meters (Innes, 1987). sitions. The distance between the highest and lowest posi- The purpose of this study was to determine the occurrence tion was about 1.5 m representing a vertical distance of of genetic differentiation between high and low intertidal about 0.24 m. Transects consisting of the four intertidal po- populations of E. linza. The results show that, despite a sitions were collected from five replicate sites each separat- free-floating spore stage, a large amount of genetic differ- ed by about 16 m horizontally along the intertidal zone. An entiation can exist between vertically separated intertidal effort was made to sample individuals from the same in- populations ofE. linza. tertidal position at each of the five sites. This was done by collecting at the edge of the water at each site as the water level fell from high to low water during the outgoing part Materials and methods of the tide cstcle. On July 21, 1982 individuals were collected at the MP Genetic differentiation in the intertidal zone was evaluated locality from high, mid and low positions in the intertidal at five localities in the Long Island 'Sound and one locality zone at three replicate sites each separated horizontally by on the Atlantic coast of Long Island (Fig. 1). At each lo- approximately 20 m. cality, single individuals of Enteromorpha linza (> 5 cm in Genetic differentiation across different intertidal posi- length) were collected at random from the highest part of tions was tested separately for each intertidal sample using the distribution of this species in the intertidal zone and at G-tests for independence between intertidal position and the edge of the water, or slightly below, at low tide. A total clone frequency (Sokal and Rohlf, 1981). The 1981 samples of 2 314 individuals were sampled with an average of 30 in- were tested for association between GOT-2 phenotype fre- dividuals per sample. In 1981 these palrs of samples from quencies and intertidal position. The 1982 samples were high and low intertidal positions were analyzed for dif- tested for association between clone frequency and in- ferences in the frequency of glutamate oxaloacetate trans- tertidal position. Cells with expected values less than one aminase (GOT-2) phenotypes. In 1982 four additional were pooled together before testing and William's cor- polymorphic loci were resolved: a second, faster migrating rection was applied to all calculated G-values (Sokal and GOT locus (GOT-I), tetrazolium oxidase (TO), amylase Rohlf, 1981). The 1982 samples from SB and MP were test- (AMY) and phosphoglucomutase (PGM). Electrophoretic ed for trends in genetic diversity with intertidal position. methods are as outlined in Innes and Yarish (1984). The Clonal diversity (Black and Johnson, 1979; Parker, 1979) observed enzyme phenotypes conformed to homozygous k was calculated as 1/~g~, where gi is the relative frequency and heterozygous patterns expected for alleles (elec- i=l tromorphs) associated with enzyme loci (Innes and Yarish, of the i th clone in a sample of k clones. Clonal evenness D.J. Innes: Genetic differentiation in the intertidal zone 11

Table 1. Enteromorpha linza. Number of individuals for two GOT- Results 2 phenotypes from high and low intertidal positions at the SB locality sampled in 1981 GOT-2 phenotype frequencies Date Intertidal N GOT-2 phenotype G position (df= 1) At the SB locality eight of nine comparisons sampled be- FF FM tween April and November showed a significant association between intertidal position and GOT-2 phenotype 4/17 Low 24 14 10 8.2** High 19 3 16 frequency (Table 1). In all eight cases the FF phenotype was most frequent in the lower intertidal zone, while the 5/3 Low 20 16 4 23.7*** High 12 0 12 FM phenotype was most common in the higher part of the intertidal zone. In the August sample the FM phenotype 5/13 Low 32 22 10 41.5"** High 32 0 32 was found to be common at both low and high intertidal positions (94 and 97%, respectively). 6/21 Low 38 32 6 20.4*** High 38 13 25 A similar result was found at the NE locality, where three of the four samples showed a significant association 7/9 Low 31 28 3 22.7*** High 24 7 17 of the FF phenotype with the low intertidal and the FM phenotype with the high intertidal (Table 2). The sample 7/26 Low 10 10 0 19.5"** High 19 4 15 collected in July had a similar frequency of the FF phenotype at both high and low intertidal positions (92 and 85%, 8/18 Low 51 3 48 0.5 ns High 39 1 38 respectively). The two samples from the MP locality also showed a 9/14 Low 29 20 9 26.1"** High 24 1 23 significant association between GOT-2 phenotypes and intertidal position (Table 2). The FM phenotype was again 11/4 Low 30 29 1 38.4*** High 11 0 11 common in the high intertidal, but the MM phenotype was the most frequent phenotype in the low intertidal of this lo- ** p < 0.01; *** p < 0.001; ns: nonsignificant cality. Finally, significant associations between GOT-2 pheno- within each sample was calculated as Clonal diversity/k, types and intertidal positions were found at the NW lo- which has a maximum value of 1.00 when the k clones are cality (Table 2). In contrast to the previous localities, the in equal frequency. FM phenotype was associated with the low intertidal and the MS phenotype was associated with the high intertidal. The FP locality was monomorphic for GOT-2 with the Laboratory experiments FF phenotype found at both high and low intertidal positions (Table 2). Six individuals, representing two GOT-2 phenotypes with different intertidal distributions, were compared for growth at two temperatures (15 ~ and 24~ Three GOT-2 FM Clone frequencies (Clone 6) individuals were collected from the high intertidal and three GOT-2 FF (one Clone 1 and two Clone 2s) Additional loci resolved in 1982 (Innes and Yarish, 1984; individuals were collected from the low intertidal at the SB Innes 1987) allowed the assignment of each individual to locality. These individuals released spores which settled one of 13 five-locus phenotypes or clones (Table 3). All and germinated in petri dishes containing 28%0 S culture three samples from SB showed a significant association be- medium as previously described (Innes and Yarish, 1984; tween intertidal position and clone frequency (Table 4). Innes, 1987). Germlings were selected for the experiment Clone 2 was consistently more frequently present in sam- when they had reached a length of about 1 mm. Forty ples from the lower intertidal compared to samples from germlings from each of the six individuals were placed in the higher intertidal. Clone 6 was more frequent in the four petri dishes (10 germlings/dish) with 10 ml of sterile high intertidal and Clone 1 more common in the low in- media (Innes and Yarish, 1984; Innes, 1987). For each intertidal samples collected in March and August. This rela- dividual, two petri dishes were placed at 15 ~ 1 C ~ and tionship was reversed in the April sample. two at 24 ~ 1 C ~ under previously described light con- There were significant associations between clone fre- ditions (Innes and Yarish, 1984). After 13 d growth, the ten quency and intertidal position for samples from NW, NE individuals in each petri dish were rinsed in distilled water and MP (Table 4). Clone 6 was associated with the high in- and oven dried for 24 h at about 60 ~ Pooled dry weights tertidal at all three localities. Clone 1 was associated with for individuals from each petri dish were determined using the low intertidal at NE and Clone 9 and 11 were associat- a Cahn (model 21) electrobalance with a sensitivity of ed with the low intertidal at NW and MP. 0.001 rag. These data were analyzed using a two factor Samples from FP and SH consisted of two or three (temperature, clone) analysis of variance (ANOVA) with clones but there were no associations with intertidal posi- subgroups (individuals) nested within clones. tion (Table 4). 12 D.J. Innes: Genetic differentiation in the intertidal zone

Table 2. Enteromorpha linza. Number of individuals for four GOT-2 phenotypes from low and high intertidal positions at the NE, MP, NW and FP localities sampled in 1981. Test of association between phenotype frequency and intertidal position. Underlined values pooled to form a new phenotype class for the test

Locality Date Intertidal N GOT-2 phenotype G position (df) FF FM MM MS

NE 4/16 Low 22 12 10 0 0 20.3*** High 22 0 22 0 0 (1) NE 5/27 Low 16 14 2 0 0 13.2"** High 16 4 12 0 0 (1) NE 7/16 Low 24 22 2 0 0 0.4 ns High 20 17 3 0 0 (1) NE 11/2 Low 28 26 2 0 0 12.5"** High 16 7 9 0 0 (1) MP 7/9 Low 16 0 i 15 0_ 43.0*** High 24 5 18 0 ! (2) MP 8/18 Low 21 1 0 20 O 48.2*** High 20 0 14 0 _6 (2) NW 7/16 Low 15 0 15 0 0 25.4*** High 16 0 3 0 13 (1) NW 8/17 Low 14 0 13 ! 0 9.4** High 5 0 6 1 _8 (l) FP 6/27 Low 16 16 0 0 0 - High 16 16 0 0 0 FP 8/20 Low 19 19 0 0 0 - High 15 5 0 0 0 FP 9/13 Low 31 31 0 0 0 - High 24 24 0 0 0

** P < 0.001; *** p < 0.01; ns: nonsignificant

Table 3. Enteromorpha linza. Thirteen clones from the Long Island from the high intertidal to the low. This was accompanied Sound as determined from five-locus phenotypes by an increased frequency of Clone 2 and one or more of Clones 1, 3 or 5 from the high to low intertidal. The pattern Locus at Site E was less clear, but the frequency of Clone 6 also GOT- 1 GOT-2 AMY PGM TO decreased from the high to low intertidal with Clone 5 increasing in frequency. 1 SS FF SS FF SS All three transects at the MP locality showed a signifi- 2 SS FF SF FF SS cant association between clone frequency and intertidal 3 SS FF SS FS SS 4 SM FF SS FF SS position (Fig. 3). Clones 9, 10, and 11 increased in frequen- 5 SF FF SF' FF SS cy from the high to low intertidal while Clones 6 and 8 de- 6 SS FM SS FF SF creased in frequency. The greatest change was observed for 7 S'S FM SS FF SF Clones 6 and 9. 8 SS MS SS FF FF 9 SS MM SS FF FF 10 S'S MM SS FS FF Genotypic diversity 11 SS MM' SS FF FF 12 SS FF SS FF FF 13 SS FM' SS FF SF At SB there was a general trend of increasing genotypic diversity from the high to low intertidal positions for the four transects in which clone frequency was significantly associated with intertidal position (Fig. 4). No obvious Intertidal transects trend between genotypic diversity and intertidal position was evident at the MP locality. Four of the five transects at the SB locality (Sites A, C, D, E), each separated by about 16 m along the beach, showed Laboratory experiments a significant association between clone frequency and intertidal position (Fig. 2). Three of the transects (A, C, D) After 13 d growth under laboratory conditions only eight showed a decrease in the relative frequency of Clone 6 out of 240 germlings (3%) died. Pooled dry weights for the D.J. Innes: Genetic differentiation in the intertidal zone 13

25q A B C 0'3 20~ _1 15- ZD t-'l 10- > 5- tm Z 1 2 3 4 Ii HIGH LOW 25 O D E 20 n~ CLONE w o 0 1 m 15" o2 o o3 10" Z 5 5" A &5

1 2 3 4 1 2 3 4 HIGH LOW HIGH LOW INTERTIDAL POSITION Fig. 2. Enteromorpha linza. Five intertidal sites (A-E) at the SB locality sampled on May 28, 1982. The graphs for each site show the number of individuals of five clones plotted for four intertidal positions (1-4). Twenty-four individuals sampled at each intertidal position except Position 2 of Site E where N=23. The following G-values (dr= 12) were obtained testing the association between clone frequency and intertidal position: Site A 33.3 (p < 0.001); Site B 7.1 (p > 0.05); Site C 54.9 (p < 0.001); Site D 59.7 (p < 0.001); Site E 26.3 (p < 0.01)

25" A B SITE ].0- D O3 20 J .8- :D 15" D .6" E 10" >

5 Z to .2- , , , to tad h 1 2 3 z 25 HIGH LOW z '1 2 3 4 O i..ij HIGH LOW > 1.0 nF 20 W CLONE I..iJ SITE a o 6 .8 O3 15 A o o 9 ZD 10 C Z *. .,. 10 ~, 11 5 .2

1 2 3 HIGH LOW HiGH LOW INTERTIDAL POSITION INTERTIDAL POSITION Fig. 3. Enteromorpha linza. Three intertidal sites (A-C) at the MP Fig. 4. Enteromorpha linza, Change in genotypic diversity, as mea- locality sampled on July 21, 1982. Graphs as in Fig. 2. Twenty- sured by evenness (see "Materials and methods"), with intertidal four individuals were sampled at each intertidal position except position. (a) Sites A, C, D, E at the SB locality (see Fig. 2). (b) Sites for Position 3 of Site C where N=28. The following G values A, B, C at the MP locality (see Fig. 3) were obtained: Site A 67.4 (p < 0.001); Site B 68.2 (p < 0.001); Site C 85.8 (p < 0.001) 14 D.J. Innes: Genetic differentiation in the intertidal zone

Table 4. Enteromorpha linza. Number of individuals of each clone in samples from low and high intertidal positions at SB, MP, NE, NW, FP and SH collected in 1982. Test of independence between clone frequency and intertidal position. Underlined values pooled to form a new clone class for the test. Clone numbers listed in Table 3. Clone 12 not present

Locality Date Intertidal N Clone G position (dr) 1 2 3 4 5 6 7 8 9 10 11 13

SB 3/15 Low 29 9 18 ! 0 _0 1 0 0 0 0 0 0 23.0*** High 21 3 3 ! 0 _1 12 1 0 0 0 0 0 (3) SB 4/14 Low 43 2 18 4 0 4 15 0 0 0 0 0 0 9.5* High 45 9 13 8 0 7 8 0 0 0 0 0 0 (4) SB 8/21 Low 64 10 8 4 0 0 35 _1 6 0 0 0 0 17.9"* High 79 7 2 0 0 0 65 0_ 5 0 0 0 0 (4) MP 6/10 Low 30 0 0 0 0 0 5 0 _1 17 1 6 0 43.5*** High 27 0 0 0 0 _1 26 0 0 0 O 0 0 (3) NE 8/24 Low 47 22 2 1 0 0 13 8 0 0 0 0 0 25.9*** High 47 9 0 0 0 0 37 1 0 0 0 0 0 (3) NW 8/24 Low 55 0 0 0 0 0 28 0 0 24 0 2 _1 9.9** High 40 _1 0 0 0 0 33 0 0 6 0 0 0 (2) FP 8/18 Low 48 31 10 7 0 0 0 0 0 0 0 0 0 0.9 ns High 48 34 10 4 0 0 0 0 0 0 0 0 0 (2) SH 4/19 Low 24 12 0 0 12 0 0 0 0 0 0 0 0 3.1 ns High 24 18 0 0 6 0 0 0 0 0 0 0 0 (1) SH 9/7 Low 24 19 3 0 _2 0 0 0 0 0 0 0 0 0.1 High 24 20 4 0 0 0 0 0 0 0 0 0 0 (1) ns

* p < 0.05; ** p < 0.01; *** p < 0.001; ns: nonsignificant

Table 5, Enteromorpha linza. Dry weights (mean-+SE for 2 repli- groups with missing individuais were adjusted to N= 10 cates) for six individuals (Clones 1, 2 and 6) after 13 d growth at ((dry weight/no, of survivors)x 10). After 13 d growth, two temperatures. The weight for each replicate represents the Clone 6 (GOT-2 FM) individuals showed a greater dry pooled weight for 10 individuals (see "Materials and methods") weight than Clones 1 and 2 (GOT-2 FF) individuals at Clone GOT-2 Dry weight (mg) both 15 ~ and 24~ (Table 5). An ANOVA on the log-trans- Phenotype formed weights showed a significant influence of clone, 15 ~ 24 ~ temperature and clone Xtemperature interaction on dry 1 FF 2.31 -t-0.20 1.34___0.16 weight (Table 6). Compared to the 15 ~ treatment, Clone 2 FF 2.30+0.26 0.98___0.09 6 individuals showed an increased growth at 24 ~ while 2 FF 1.39___0.21 0.63+0.06 Clones 1 and 2 showed a decrease in growth (Table 5). A 6 FM 4.79 _+ 0.02 7.74___ 0.08 nonsignificant individual within clone• in- 6 FM 5.18_+0.06 8.73_+0.56 teraction indicated that the effect of temperature was the 6 FM 4.45_+0.20 7.13+0.68 same for the different individuals within each clone.

Discussion Table 6. Enteromorpha linza. Summary of analysis of variance on log-transformed dry weights after 13 d growth at two temperatures These results demonstrate that genetic differentiation oc- (15 ~ and 24 ~ for different clones (see Table 5) curs within the intertidal zone for some populations of En- Source DF MS F P teromorpha linza. Although the pattern of differentiation was generally consistent within localities sampled during Clone (C) 1 2.5350 49.0 < 0.005 different seasons as well as different years, there were dif- Individuals within 4 0.05165 19.87 < 0.001 ferences in the pattern of intertidal differentiation between clones C (I) some of the localities. Where intertidal differentiation oc- Temperature (T) 1 0.0231 28.88 < 0.01 curred, Clone 6 was usually associated with the high in- TX C 1 0.4525 565.63 < 0.001 tertidal, but the clonal composition of the low intertidal C (I) x T 4 0.0008 0.31 NS varied among localities. For example, the low intertidal of SB and NE was dominated by Clones 1 and 2, while Clone Error 12 0.0026 9 was the most common clone at NW and MP. This differ- D.J. Innes: Genetic differentiation in the intertidal zone 15 entiation in the low intertidal among these localities was linza, as measured by the evenness in relative frequency associated with differences in habitat salinity (Innes, 1987). among more than one clone, generally decreased from the Dispersal rates between high and low intertidal popula- low intertidal into the high intertidal for transects at the SB tions of Enteromorpha linza are not known. However, the locality. In this case the high intertidal zone apparently small distances between intertidal populations combined represents an extreme environment, favoring one or a few with a free-floating spore stage would suggest that selection well adapted genotypes. may be important in producing the observed patterns of The genetic structure of clonal reproducing En- differentiation. Differentiation between plant populations teromorpha linza in the Long Island Sound is dominated by adapted to different habitats, but connected by migration, factors operating on a microgeographic scale. Genetic dif- has been well documented (Bradshaw, 1971; Caisse and ferentiation across an environmental gradient of a few Antonovics, 1978). The differential growth-response of meters in the intertidal zone can be greater than differen- Clones 2 and 6 under different temperature conditions pro- tiation between populations separated by several to tens of vides additional evidence for adaptive differentiation in the kilometers (Innes, 1987). Microgeographic differentiation intertidal zone. Reciprocal transplant studies (Ayre, 1985) has occurred despite a dispersing spore stage and between and experimental introductions of different clones to dif- areas connected by water with no obvious barriers to dis- ferent intertidal positions would assist in determining the persal. The results of this study suggest that differential se- influence differences in growth and survival among clones lection among clones in a heterogeneous environment is have for maintaining the observed differentiation in the in- one factor maintaining the observed microgeographic dif- tertidal zone. It is also possible that limited recruitment ferentiation. Additional studies are needed to determine and the regeneration of individual plants from microscopic the relative importance of recruitment and competitive dif- holdfasts could both contribute to the maintenance of differences among the clones in maintaining differentiation in ferentiation within the intertidal zone (Innes, 1987). the intertidal zone. A recent study of the intertidal alga Fucus disticus also found evidence for genetic differentiation in the intertidal Acknowledgements. I thank R. K. Koehn for his continuing zone (Sideman and Mathieson, 1983). A dwarf form of this support and encouragement. Many discussions with J. G. species was more common in the high than the low in- Hall, L. Harshman, M. E. Saks and A. J. Zera contributed tertidal. Growth experiments under co{nmon en- to the improvement of this research. C. Yarish was very vironmental conditions confirmed that this morphological helpful in numerous ways including collecting and iden- difference, as well as earlier maturing for the dwarf form, tifying different species of green algae. I thank P. D. H. He- were genetically based. It should be noted that F. disticus is bert, G.J. Hechtel, R.K. Koehn, J.S. Levinton, R.D. monoecious and self-fertilizing, therefore populations may Ward, G.C. Williams and C. Yarish for their comments. consist of inbred lines with some lines adapted to different This research was supported by grant DEB 7908802 from conditions. Populations of some self-fertilizing plant the National Science Foundation to R.K. Koehn. I also species have been shown to consist of inbred lines adapted wish to acknowledge a postgraduate scholarship from the to different moisture conditions (Clegg and Allard, 1972; NSERC of Canada and a Grant-in-Aid of Research award Hamrick and Allard, 1972, 1975). from Sigma Xi. The observation of strong differentiation over short distances in Enteromorpha linza is in contrast with studies of genetic variation in invertebrate species from the intertidal Literature cited zone (Koehn etal., 1973; Tracey etal., 1975; Flowerdew and Crisp, 1976; Black and Johnson, 1981; Johnson and Ayre, D. J.: Localized adaptation of clones of the sea anemone Ac- Black, 1982). Large differences between the environments tinia tenebrosa. Evolution 39, 1250-1260 (1985) Biebl, R.: Vergleichende Untersuchungen zur Temperaturresistenz of the high and low intertidal positions do not necessarily von Meeresalgen entlang der pazifischen Kt~ste Nordamerikas. result in large amounts of genetic differentiation. Many in- Protoplasm 69, 61-83 (1970) tertidal species may survive environmental variation in the Black, R. and M. S. Johnson: Asexual viviparity and population intertidal zone through phenotypic plasticity rather than genetics ofActinia tenebrosa. Mar. Biol. 53, 27-31 (1979) through genetic adaptations to different conditions (Black Black, R. and M. S. Johnson: Genetic differentiation independent of intertidal gradients in the pulmonate limpet Siphonaria kur- and Johnson, 1981). Alternatively, genetic differentiation racheensis. Mar. Biol. 64, 79-84 (1981) in the intertidal zone may not be associated with enzyme Bradshaw, A.D.: Plant evolution in extreme environments. In: loci. Ecological genetics and evolution, pp 20-50. Ed by R. Creed. The intertidal zone has previously been thought of as a Oxford: Blackwell Scientific Publications 1971 Caisse, M. and J. Antonovics: Evolution in closely adjacent plant heterogeneous environment which could support a higher populations. IX. Evolution of reproductive isolation in clinal level of genetic variability than the subtidal environment populations. Heredity 40, 371-384 (1978) (Levinton, 1973; Black and Johnson, 198l; Johnson and Chapman, A. R. O.: A critique of prevailing attitudes towards the Black, 1982). Observations of allozyme variation in several control of seaweed zonation on the sea shore. Bot. mar. 16, 80-82 (1973) invertebrate species has both supported (Levinton, 1973; Clegg, M. T. and R. W. Allard: Patterns of genetic differentiation Mathers, 1975) and contradicted (Levinton, 1975; Wilkins, in the slender wild oat species Arena barbata. Proc. natl Acad. 1977) this hypothesis. Genetic diversity in Enteromorpha Sci. 69, 1820-1824 (1972) 16 D.J. Innes: Genetic differentiation in the intertidal zone

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