Regulation of gamete release in the economic brown seaweed <Emphasis Type="Italic">Hizikia fusiforme</Emphasis> (Phaeophyta)

Gamete release is an essential event in artificial seeding of the economic brown seaweed, Hizikia fusiforme. Mass egg release occurred in the dark, with few eggs being discharged in the light. Release of eggs was elicited with eight practical salinity units (one PSU ≡ 1 g sea salts l⁻¹) and was inhibited by salinity levels >32 PSU. Egg release was optimal at 23 °C, and was decreased by 72% in agitated seawater compared to unstirred seawater. Inhibitors of photosynthesis and ions channels suppressed egg release, indicating that this process was physiologically associated with photosynthetic activity and ion transport.     

Experimental sexual selection in Chlamydomonas

Sexual and asexual lines of the unicellular chlorophyte Chlamydomonas reinhardtii were propagated for about 100 sexual cycles and 1000 vegetative cycles in contrasted environments, liquid and solid growth media, in order to generate divergent natural and sexual selection. Sexual lines were transferred by many zygotes or by a single zygote in each sexual generation. By the end of the experiment zygote production was in the order sexual mass-transfer>sexual single-zygote>asexual>ancestor. The direct response to sexual selection was large, with zygote production increasing by about two orders of magnitude, mainly because mating had become spontaneous instead of being invoked by nitrogen starvation. Asexual lines became sexually sterilized by the fixation of a single mating type. Sexual selection caused a radical shift in the gender system, with homothallism spreading to high frequency in all sexual lines of this normally heterothallic species. This may have been caused by the transposition of a mating-type gene to an autosome. No substantial degree of environment-specific mating evolved, however, and thus no sexual isolation indicative of incipient speciation. It is possible that selection experiments of this kind are unlikely to induce sexual isolation because mating-type genes evolve in a saltatory fashion.     

鹅掌楸配子选择与雄性繁殖适合度

在多父本等量花粉混合授粉的交配实验设计基础上, 利用SSR分子标记对其子代进行父本分析, 研究鹅掌楸的配子选择与雄性繁殖适合度。参试亲本为鹅掌楸2个种的5个单株（鹅掌楸（Liriodendron chinense）2株, 即FY和LS; 北美鹅掌楸（L. tulipifera）3株, 即LYS、MSL和NK）。结果表明： 鹅掌楸的配子选择个体间差异较大。作为母本, NK和LYS倾向于选择异种雄配子, 而MSL和LS则倾向于选择同种雄配子; 在同种雄配子的选择中, NK、LYS和LS倾向于自交, 而MSL则倾向于异交。以北美鹅掌楸为母本时, 北美鹅掌楸与鹅掌楸的雄性繁殖贡献率分别为45.5%和54.5%, 北美鹅掌楸的雄性繁殖适合度为鹅掌楸的0.556倍。以鹅掌楸为母本时, 二者的繁殖贡献率分别为15.6%和84.4%, 北美鹅掌楸的雄性繁殖适合度为鹅掌楸的0.123倍。总体上看, 鹅掌楸的雄性繁殖适合度高于北美鹅掌楸, 鹅掌楸与北美鹅掌楸均表现为自交亲和。     

ASYMMETRY OF EYESPOT AND MATING STRUCTURE POSITIONS IN ULVA COMPRESSA (ULVALES,CHLOROPHYTA) REVEALED BY A NEW FIELD EMISSION SCANNING ELECTRON MICROSCOPY METHOD1

Gametes of the marine green alga Ulva compressa L. are biflagellate and pear shaped, with one eyespot at the posterior end of the cell. The species is at an early evolutionary stage between isogamy and anisogamy. In the past, zygote formation of green algae was categorized solely by the relative sizes of gametes produced by two mating types (+ and ?). Recently, however, locations of cell fusion sites and/or mating structures of gametes have been observed to differ between mating types in several green algae (asymmetry of cell fusion site and/or mating structure positions). To use this asymmetry for determining gamete mating type, we explored a new method, field emission scanning electron microscopy (FE‐SEM), for visualizing the mating structure of U. compressa. When gametes were subjected to drying stress in the process of a conventional critical‐point‐drying method, a round structure was observed on the cell surfaces. In the mating type MGEC‐1 (mt⁺), this structure was located on the same side of the cell as the eyespot, whereas it was on the side opposite the eyespot in the mating type MGEC‐2 (mt^?). The gametes fuse at the round structures. TEM showed an alignment of vesicles inside the cytoplasm directly below the round structures, which are indeed the mating structures. Serial sectioning and three‐dimensional construction of TEM micrographs confirmed the association of the mating structure with flagellar roots. The mating structure was associated with 1d root in the MGEC‐1 gamete but with 2d root in the MGEC‐2 gamete.     

Role of odour compounds in the attraction of gamete vectors in endophytic Epichloë fungi

Grass-infecting Epichlo? endophytes (Ascomycota, Calvicipitaceae) depend on Botanophila flies for gamete transfer, while fly larvae feed and develop on the fertilized fungal fruiting structures. Flies are known to be attracted by volatile signals, but the exact mechanisms of chemical communication and the degree of specialization are unknown. Headspace samples collected from five different Epichlo? species were analysed with respect to physiologically active substances using Botanophila flies. In field bioassays using synthetic compounds, their attractiveness and the specificity of the Epichlo?-Botanophila attraction were investigated. The identification of a new natural product, methyl (Z)-3-methyldodec-2-enoate, attracting Botanophila flies is reported here, and chokol K is confirmed as an attractive compound. Different blends of the two compounds attracted Botanophila flies under field conditions, but the three fly taxa present at the study site showed no preference for specific blends of volatiles. Chemical communication in the Epichlo?-Botanophila system relies on a few specific compounds, known as a communication system with 'private channels'. Although ratios of emitted compounds vary in different Epichlo? species, this seems not to lead to specialized attraction of Botanophila flies. Low selective pressure for specialization may have maintained a more generalist interaction between fungi and flies.     

Temporal variation in the pollen:ovule ratios of Clarkia (Onagraceae) taxa with contrasting mating systems: field populations

Among plants, pairs of selfing vs. outcrossing sister taxa provide interesting systems in which to test predictions concerning the magnitude and direction of temporal changes in sex allocation. Although resource availability typically declines towards the end of the growing season for annual taxa, temporal changes in mating opportunities depend on mating system and should change less in selfing taxa. Consequently, given that the pollen:ovule (P:O) ratio of flowers reflects the investment in (and potential fitness pay-off due to) male vs. female function, we predicted that the P:O ratio should also be less variable among and within selfers than in closely related outcrossers. To test these predictions, we measured temporal changes in sex allocation in multiple field populations of two pairs of sister taxa in the annual flowering plant genus Clarkia (Onagraceae). In the outcrossing Clarkia unguiculata and the selfing Clarkia exilis, ovule production declined similarly from early to late buds, whereas pollen production remained constant or increased in the outcrosser but remained constant or decreased in the selfer. Consequently, the P:O ratio increased within unguiculata populations but marginally increased or stayed constant in exilis populations. In all populations of the selfing Clarkia xantiana spp. parviflora and the outcrossing C. x. spp. xantiana, both ovule and pollen production per flower declined over time. The effects of these declines on the P:O ratio, however, differed between subspecies. In each xantiana population, the mean P:O ratio did not differ between early and late flowers, although individuals varied greatly in the direction and magnitude of phenotypic change. By contrast, parviflora populations differed in the mean direction of temporal change in the P:O ratio. We found little evidence to support our initial predictions that the P:O ratio of the selfing taxa will consistently vary less than in outcrossing taxa.     

GAMETE INTERACTIONS AND GENETIC DIFFERENTIATION AMONG THREE SYMPATRIC POLYCHAETES

The evolution of gamete incompatibility between free-spawning marine invertebrate species has been explained by three hypotheses: (1) independent divergence at gamete recognition loci; (2) selection against hybrids; and (3) a process of sexual selection involving polymorphic gamete recognition loci (Metz and Palumbi 1996). The first two hypotheses predict that gamete incompatibility appears only after gene flow has been halted for other reasons and the third that gamete incompatibility appears simultanously with blocks to gene flow. Here I show that gametes of three sympatric polychaetes in the genus Arctonoe are compatible in all crosses, over a broad range of gamete concentrations and contact times. Although at least some hybrid crosses produce fertile adults, allozyme and mitochondrial DNA sequence data indicate that the three species do not regularly exchange genes. These data are consistent with predictions of the first two hypotheses for the evolution of gamete incompatibility, but allow rejection of the third hypothesis. Gametes of the three species are compatible despite estimated divergence times of 1–3 M.Y.B.P.; in several other marine invertebrates, divergence times of the same magnitude are associated with asymmetric or complete gamete incompatibility. It appears likely that segregation of symbiotic adults on their respective host species restricts mating opportunities, and thus gene flow, among Arctonoe species.     

RECENT ADVANCES IN FERTILIZATION ECOLOGY OF MACROALGAE1

Our understanding of natural patterns of fertilization in seaweeds has increased substantially over the last 10 years due to new approaches and methods to characterize the nature and frequency of fertilization processes in situ, to recognize the conditions and mechanisms enhancing fertilization success, and to anticipate population and community consequences of the patterns of natural fertilization. Successful reproduction in many species depends on a delicate juxtaposition of abiotic and biotic conditions. Important abiotic factors are those triggering gamete release (e.g. single or interacting effects of light quality and water movement) and those affecting gamete viability or concentrations (e.g. salinity effects on polyspermy blocks; gamete dilution due to water movement). Examples of important biotic components are synchronous gamete release, efficiency of polyspermy‐blocking mechanisms, population density of sexually fertile thalli, interparent distances, and male‐to‐female ratios. Field data indicate fertilization frequencies of 70%–100% in broadcasting‐type seaweeds (e.g. fucoids) and 30%–80% in brooding‐type (red) algae. Red algal values are higher than previously thought and challenge presently accepted explanations for their complex life histories. Important population and community questions raised by the recent findings relate to the magnitude of gene flow and exchange occurring in many micropopulations that seemingly breed during periods of isolation, the physiological basis and population effects of male‐to‐male competition and sexual selection during fertilization of brooding seaweeds, and the effects of massive gamete release, especially in holocarpic seaweeds, on benthic and planktonic communities. Comparative studies in other algal groups are now needed to test the generality of the above patterns, to provide critical pieces of information still missing in our understanding of natural fertilization processes, and to elucidate the evolutionary consequences of the different modes of reproduction (e.g. brooders vs. broadcasters).