Enrique A. Martínez

RECOVERY AND GENETIC DIVERSITY OF THE INTERTIDAL KELP LESSONIA NIGRESCENS (PHAEOPHYCEAE) 20 YEARS AFTER EL NIÑO 1982/831

Massive mortality in kelp beds of the Pacific coasts of North and South America was caused by the rise in surface seawater temperature during the El Niño Southern Oscillation (ENSO) event of 1982/83, the strongest in the four and half previous centuries. In northern Chile a stretch of 600 km of coastline showed massive mortality of the intertidal kelp species Lessonia nigrescens Bory, of which only a few individuals managed to survive. Kelps and their associated biodiversity recovered but kelp beds re‐colonization in general was variable in time and space seemingly very slow along northern Chilean coasts. Here we show, effectively, that northward re‐colonization advanced less than 60 km in 20 years. Conversely, kelp beds of the Northern Hemisphere recovered 300 km in only six months after the same ENSO event. Genetic diversity in the two most affected populations of L. nigrescens shows half of the heterozygosity and polymorphism with respect to that observed in six non affected populations. In addition, geographically separated populations seem highly isolated as evidenced by high and significant fixation indices (all FST values over 0.4).

Hierarchical spatial structure and discriminant analysis of genetic diversity in the red alga Mazzaella laminarioides (Gigartinales, Rhodophyta)

Our study of the genetic structure of Mazzaella laminarioides (Bory) Fredericq (Gigartinales) in the central Chilean region documented a high level of genetic diversity based on random amplified polymorphic DNA (RAPD) markers and indicated the occurrence of significant genetic structure at different spatial scales. A total of 288 haploid gametophytes was analyzed with 17 polymorphic RAPD bands, which produced 202 distinct multilocus genotypes. Within stands, mean gene diversity ranged from 0.210 to 0.249 and no significant linkage disequilibrium could be detected among pairs of alleles, revealing that recombination (sexual reproduction) regularly shuffles the genes at that scale. Analysis of molecular variance within stands (less than 30 m) showed that the structure was very low, only marginally significant, and did not increase with increasing hierarchical levels at this lowest spatial scale. In contrast, at a larger spatial scale (among stands, from 5 to 60 km), increasing geographical distance seemed to account for increasing isolation between populations even if natural barriers, such as sandy beaches or river estuaries, may play a role in such isolation. Moreover, the strong genetic differentiation occurring between locations separated by 60 km allowed the assignment of individuals to their original population through a canonical discriminant analysis. This approach further allowed the identification of potential recent migrants from one population to the other. Thus, in species like M. laminarioides for which the dominance of RAPD markers can be avoided by selecting haploid individuals, RAPD analysis appeared to be specially appropriate for the study of genetic differentiation.

REDUCED GENETIC DIVERSITY AND INCREASED POPULATION DIFFERENTIATION IN PERIPHERAL AND OVERHARVESTED POPULATIONS OF GIGARTINA SKOTTSBERGII (RHODOPHYTA, GIGARTINALES) IN SOUTHERN CHILE1

This study assesses two hypotheses on the genetic diversity of populations of Gigartina skottsbergii Setchell et Gardner (Rhodophyta, Gigartinales) at the border of the species distribution: 1) peripheral populations display a reduced genetic diversity compared with central populations, and 2) genetic differentiation is higher among peripheral than among central populations. Two peripheral and four central populations were sampled along the Chilean coast and 113 haploid individuals were analyzed using 17 random amplification of polymorphic DNA loci. The genetic diversity was estimated by allele diversity (He), allele richness (Â), and the mean pair‐wise differences among multilocus genotypes. All three estimates consistently and significantly indicated a lower genetic diversity within the peripheral than within the central populations. Genetic differentiation between the two peripheral populations was stronger (FST=0.35) than between central populations at similar spatial scales (FST ranging from 0 to 0.25). In addition, it appeared from the distribution of pair‐wise differences that peripheral populations are in demographic expansion after a recent bottleneck. The results are discussed in the specific context of potential overharvesting of these wild populations.

Effects of filter-feeders and grazers on algal settlement and growth in mussel beds

Abstract Macroalgae and mussels are common users of the often limiting primary space on mid- and low-intertidal rocky surfaces. As a result most studies of their interactions have focussed primarily on resource competition. Yet, frequently a diversity of algae is found coexisting with mussels — neither asphyxiating them nor being outcompeted by them. In this study, we look for other sets of ecologically important interactions between mussels and algae in the habitats where they coexist. Experimental studies showed that mussels ingest and digest algal spores, affect the pattern of algal settlement and protect the algal germlings from desiccation. Probably, they also fertilize the growing thalli. Small grazers among the mussels select the type of algae surviving in the bed and together with the abiotic environment control algal growth and abundance. Reproductive output, antiherbivore escape and defense mechanisms and tolerance to desiccation appear to be the most important algal adaptations for survival in this type of habitat. The resulting flora found among mussels is a function of all these interactions and of the age and history of the individual mussel bed.

MOLECULAR IDENTIFICATION OF TWO SIBLING SPECIES UNDER THE NAME GRACILARIA CHILENSIS (RHODOPHYTA, GRACILARIALES)1,3

Molecular markers belonging to three different genomes, mitochondrial (cox2‐3 spacer), plastid (RUBISCO spacer), and nuclear (internal transcribed spacer 1), were used to compare Gracilaria chilensis samples collected along the Chilean coast with samples ascribed to G. chilensis from the West Pacific Ocean (New Zealand and Australia). Our data are in agreement with previous studies suggesting two sibling species currently going under the name G. chilensis that co‐occur in New Zealand. One of these, a New Zealand sample previously examined by Bird and others in 1990, is conspecific with G. chilensis from Chile. Finally, our results demonstrate clearly that most of the sequences in GenBank reported as G. chilensis are based on misidentified material.

SIZE HIERARCHY AND THE −3/2 “POWER LAW” RELATIONSHIP IN A COALESCENT SEAWEED1

The size structure of plant populations has been characterized by two descriptors, size‐density relations and size inequality. In even aged natural or experimental populations, the logarithm of mean size is a negative function of population density, with a slope of –3/2 and a maximum intercept of 4.3. Size inequality, measured by the Gini coefficient (G.C.), ranges from maximum size homogeneity (G.C. = 0) to maximum inequality (G.C. = 1). These two descriptors have been applied mainly to land plants, and only once have they been empirically related, suggesting that the onset of self‐thinning (increase in mean size by decreasing density) reduces size inequality. We applied these descriptors and tested their empirical relations within populations of Iridaea laminarioides Bory, a red alga that, as other members of the Gigartinales and Gracilariales, exhibits coalescence of germinating spores, forming a clump of fronds with a single basal disc. Results indicate 1) lack of fit to the self‐thinning rule and transgression of the limit slope, 2) high size inequality values, 3) absence of a significant negative correlation between mean size and Gini values, 4) spatial ordering in the distribution of size inequality, with fronds decreasing in size towards the periphery of the coalescing discs, and 5) reproduction occurring mostly in the large sized fronds. In I. laminarioides transgression of the self‐thinning slope and high size inequality seems to result from spore coalescence and a growth pattern that allows the coalescing discs to grow into physiologically integrated fronds, the larger (and central) of which become reproductive. Therefore, rather than being considered exceptional, we suggest that transgression to the limit slope and high size inequality should be common among seaweeds with coalescing spores and physiological integration of individuals.

Selective mortality on haploid and diploid microscopic stages of Lessonia nigrescens Bory (Phaeophyta, Laminariales)

Abstract In seaweeds, potential selective events on juveniles are particularly important because the highest mortality rates usually affect these microscopic stages rather than the macroscopic ones. This study evaluates the degree to which the mortality induced by herbivory and wave impact are selective on the size (mean and whole distribution) of haploid, gametophytic, and of diploid, early sporophytic stages of the intertidal kelp Lessonia nigrescens . In theory, in species with a complex life history, the potential effect of selective mortality on haploid and diploid individuals may range between two extremes. One of inhibition, where the same feature, conferring higher survival in one phase, may cause the opposite effect in the following phase. The other extreme is synergism, where selection operates similarly among the two alternating life history phases. Controlled experiments showed that herbivorous snails had a more significant negative impact on haploid gametophytes than on the slightly larger, diploid, sporophytes. Conversely, separate experiments showed that wave impact eliminated more efficiently the larger diploid plantules than the smaller haploid ones. Size selectivity by both mortality agents occurred only on the microscopic diploid stages, when plantule sizes were longer than 20 μm. Directionality of selection indicated that larger individuals, with more developed adhesive rhizoids, had higher survival rates. The directional selection on diploid plants is neither reinforced, or cancelled out, in the alternate haploid stages. Thus, no inhibitory or synergetic effects were observed for the directional selection of size between these two life history phases. These results suggest that, compared to organisms with direct life cycles, those having complex life histories, with independent phases, have at least one more restriction for the occurrence of evolutionary change. Namely, that of equivalent expression and response of involved traits on both alternate haploid and diploid phases to the same putative selective agents.

CHARACTERIZATION OF GENETIC MARKERS LINKED TO SEX DETERMINATION IN THE HAPLOID‐DIPLOID RED ALGA GRACILARIA CHILENSIS1

Bulk segregant analysis, random amplified polymorphic DNA (RAPD), and sequence characterized amplified region (SCAR) methods were used to identify sex‐linked molecular markers in the haploid‐diploid rhodophyte Gracilaria chilensis C. J. Bird, McLachlan et E. C. Oliveira. One hundred and eighty 10 bp primers were tested on three bulks of DNA: haploid males, haploid females, and diploid tetrasporophytes. Three RAPD primers (OPD15, OPG16, and OPN20) produced male‐specific bands; and one RAPD primer (OPD12), a female‐specific band. The sequences of the cloned putative sex‐specific PCR fragments were used to design specific primers for the female marker SCAR‐D12‐386 and the male marker SCAR‐G16‐486. Both SCAR markers gave unequivocal band patterns that allowed sex and phase to be determined in G. chilensis. Thus, all the females presented only the female band, and all the males only the male band, while all the tetrasporophytes amplified both male and female bands. Despite this sex‐specific association, we were able to amplify SCAR‐D12‐386 and SCAR‐G16‐486 in both sexes at low melting temperature. The differences between male and female sequences were of 8%–9% nucleotide divergence for SCAR‐D12‐386 and SCAR‐G16‐486, respectively. SCAR‐D12‐386 and SCAR‐G16‐486 could represent degenerated or diverged sequences located in the nonrecombining region of incipient sex chromosomes or heteromorphic sex chromosomes with sequence differences at the DNA level such that PCR primers amplify only one allele and not the other in highly specific PCR conditions. Seven gametic progenies composed of 19 males, 19 females, and the seven parental tetrasporophytes were analyzed. In all of them, the two SCAR markers segregated perfectly with sexual phenotypes.

FACTORS ASSOCIATED WITH HOST SPECIFICITY IN SPOROCLADOPSIS NOVAE‐ZELANDIAE (CHLOROPHYTA)1

This study tests the hypotheses that substrate stability and rugose microtopography are the main factors that determine the tissue‐specific epiphytism in the chlorophyte Sporocladopsis novae‐zelandiae Chapman. Both in vitro and field experiments showed that the epiphyte did not develop on stable algal surfaces nor on artificial (smooth or rugose) substrata. In field experiments, however, other macroalgae settled more abundantly on rugose substrata. Similarly, our field experiments did not support the hypothesis that the physical environment provided by the spatial location of the host in the intertidal or subtidal was a relevant factor associated with specificity. In contrast, our laboratory experiments suggested that some component of the host cell wall might trigger the formation of penetrating rhizoids. These did not develop in isolated cultures of the epiphyte but were detected when the epiphyte contacted its natural host. Also, preliminary results showed that rhizoid‐like structures developed, although at very low frequency, only in plants grown in culture medium containing a crude extract of sorus cortical tissue.