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Female nuclear speeds before and after male nucleus entry into the trichogyne. Analysis of the time-lapse Movies S3a and b. (A) Plot; (B) numerical values. The speed of the green-labeled H1-sGFP female nuclei of the trichogyne and of the vegetative hyphae in the same field of view were measured at every time frame before (Movie S3a) and after (Movie S3b) entry of a male nucleus into the trichogyne. S.D, standard deviation; N nuclei , number of analyzed nuclei in the field of view; N speed , number of nuclei speeds measured. Results from the ANOVA (on log values) indicate that the effect of target site is significant (P , 0.001), with larger values in hyphae (1.56 6 1.63, n = 683) than in trichogyne (0.92 6 0.94, n = 749); likewise, the effect of time of observation is significant (P , 0.001), with larger values before (1.42 6 1.73, n = 569) than after (1.10 6 1.01, n = 863); these two effects are independent of one another as confirmed by the nonsignificant interaction (P = 0.431). Pairwise t tests with pooled SD indicate that all pairs of means were different at the 5% level.
Source publication
Sexual reproduction is a key process influencing the evolution and adaptation of animals, plants, and many eukaryotic microorganisms, such as fungi. However, the sequential cell biology of fertilization and the associated nuclear dynamics after plasmogamy are poorly understood in filamentous fungi. Using histone-fluorescent parental isolates, we tr...
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Context 1
... Upon cessation of trichogyne growth at its targeted conidium, the anterograde movement of female nuclei switched to an oscillatory motion and then became visually immobilized 4 min before the entry of the first male nucleus (t = 1 h 48 min). Nucleus speed measurements revealed that female nucleus movements did not fully stop but were reduced (Fig. 3). For the sake of simplicity, we will refer to this phase as female nuclear immobilization. These female nuclear dynamics suggest stepwise signaling events anticipating the entry of the male nucleus. Female nuclei remained immobilized while male nuclei migrated up the trichogyne. Finally, 56 min after the first male RFP nucleus entered ...
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... female GFP nuclei synchronously began moving backward (retrograde movement) up the trichogyne (t = 2 h 44 min). These female nuclear dynamics were consistent with every fertilization event analyzed (n = .10). We quantified female nucleus mobility before and after entry of male nuclei into trichogynes for at least one fertilization event (Fig. 3). By tracking nuclei in Movies S3a (before entry) and S3b (after entry), we measured the speed of GFP female nuclei within trichogynes after trichogyne growth had ceased at the conidium and after female nuclei were immobilized. These dynamics were compared to the mobility of female nuclei located in surrounding hyphae, likely vegetative ...
Context 3
... entry), we measured the speed of GFP female nuclei within trichogynes after trichogyne growth had ceased at the conidium and after female nuclei were immobilized. These dynamics were compared to the mobility of female nuclei located in surrounding hyphae, likely vegetative ones since initial nucleus behavior in these hyphae were different ( Fig. 3 and Movies S3a and ...
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... average speed of hyphal female nuclei was significantly higher (2.04 6 2.36 mm/ min) than that of trichogyne female nuclei (1.03 6 1.00 mm/min; Fig. 3; P , 10 23 ) prior to male nucleus entry. Strikingly, the speed of both hyphal and trichogyne female nuclei significantly decreased after male nucleus entry, to 1.33 6 1.07 mm/min (P , 10 23 ) and 0.82 6 0.86 mm/min (P , 10 23 ), respectively. These data confirmed the immobilization of female nuclei observed immediately prior to entry ...
Citations
... This hormonal chemotropism is controlled by the mating type genes on a pheromone/receptor recognition basis and occurs only between cells of opposite mating type in heterothallic species (Bistis, 1983). The contact between the trichogyne and the spermatia triggers cell fusion, i.e. plasmogamy, after which the spermatia nucleus migrates across the trichogyne into the ascogonium (Brun et al., 2021). Plasmogamy is controlled by mating type genes (Bistis, 1996;Peraza-Reyes and Malagnac, 2016). ...
... Neurospora crassa (Maheshwari, 1999). In these two species, however, microconidia designate the male gametes, are able to germinate, and are distinct from macroconidia (often called conidia) which are asexual spores but can also be fertilizing elements (Brun et al., 2021); hence both microconidia and macroconidia are spermatia in B. cinerea and N. crassa. ...
Sexual reproduction in Ascomycetes is well described in several model organisms such as Neurospora crassa or Podospora anserina. Deciphering the biological process of sexual reproduction (from the recognition between compatible partners to the formation of zygote) can be a major advantage to better control sexually reproducing pathogenic fungi. In Pyricularia oryzae, the fungal pathogen causing blast diseases on several Poaceae species, the biology of sexual reproduction remains poorly documented. Besides the well-documented production of asexual macroconidia, the production of microconidia was seldom reported in P. oryzae, and their role as male gamete (i.e., spermatia) and in male fertility has never been explored. Here, we
characterised the morphological features of microconidia and demonstrated that they are bona fide spermatia. Contrary to macroconidia, microconidia are not able to germinate and seem to be the only male gametes in P. oryzae. We show that fruiting body (perithecium) formation requires microconidia to get in contact with mycelium of strains of opposite mating type, to presumably fertilise the female gametes.
... The physiological mechanisms occurring during sexual reproduction have been described in some model ascomycetes (e.g. Neurospora crassa; Brun et al., 2021), but the interaction with the host plant remains poorly understood for biotrophic and hemibiotrophic plant pathogens. ...
Little is known about the impact of host immunity on sexual reproduction in fungal pathogens. In particular, it is unclear whether crossing requires both sexual partners to infect living plant tissues. We addressed this issue in a three-year experiment investigating different scenarios of Zymoseptoria tritici crosses according to the virulence (‘vir’) or avirulence (‘avr’) of the parents against a qualitative resistance gene. Co-inoculations (‘vir × vir’, ‘avr × vir’, ‘avr × avr’) and single inoculations were performed on a wheat cultivar carrying the Stb16q resistance gene (Cellule) and a susceptible cultivar (Apache), in the greenhouse. We assessed the intensity of asexual reproduction by scoring disease severity, and the intensity of sexual reproduction by counting the ascospores discharged from wheat residues. As expected, disease severity was more intense on Cellule for ‘vir × vir’ co-inoculations than for ‘avr × vir’ co-inoculations, with no disease for ‘avr × avr’. However, all types of co-inoculation yielded sexual offspring, whether or not the parental strains caused plant symptoms. Parenthood was confirmed by genotyping (SSR markers), and the occurrence of crosses between (co-)inoculated and exogenous strains (other strains from the experiment, or from far away) was determined. We showed that symptomatic asexual infection was not required for a strain to participate in sexual reproduction, and deduced from this result that avirulent strains could be maintained asymptomatically “on” or “in” leaf tissues of plants carrying the corresponding resistant gene for long enough to reproduce sexually. In two of the three years, the intensity of sexual reproduction did not differ between the three types of co-inoculation in Cellule, suggesting that crosses involving avirulent strains are not anecdotal. We discuss the possible mechanisms explaining the maintenance of avirulence in Z. tritici populations and the potential impact of particular resistance deployments such as cultivar mixtures for limiting resistance breakdown.
... This hormonal chemotropism is controlled by the mating type genes on a pheromone/receptor recognition basis and occurs only between cells of opposite mating type in heterothallic species (Bistis, 1983). The contact between the trichogyne and the spermatia triggers cell fusion, that is, plasmogamy, after which the spermatia nucleus migrates across the trichogyne into the ascogonium (Brun et al., 2021). Plasmogamy is controlled by mating-type genes (Bistis, 1996;Peraza-Reyes & Malagnac, 2016). ...
... Microconidia were proved to be spermatia in several Ascomycete species, including Botrytis cinerea (Fukumori et al., 2004) and Neurospora crassa (Maheshwari, 1999). In these two species, however, microconidia designate the male gametes, are able to germinate, and are distinct from macroconidia (often called conidia) which are asexual spores but can also be fertilising elements (Brun et al., 2021); hence both microconidia and macroconidia are spermatia in B. cinerea and N. crassa. ...
Sexual reproduction in Ascomycetes is well described in several model organisms such as Neurospora crassa or Podospora anserina. Deciphering the biological process of sexual reproduction (from the recognition between compatible partners to the formation of zygote) can be a major advantage to better control sexually‐reproducing pathogenic fungi. In Pyricularia oryzae, the fungal pathogen causing blast diseases on several Poaceae species, the biology of sexual reproduction remains poorly documented. Besides the well‐documented production of asexual macroconidia, the production of microconidia was seldom reported in P. oryzae, and their role as male gamete (i.e. spermatia) and in male fertility has never been explored. Here we characterized the morphological features of microconidia and demonstrate that they are bona fide spermatia. Contrary to macroconidia, microconidia are not able to germinate and seem to be the only male gametes in P. oryzae. We show that fruiting body (perithecium) formation requires microconidia to get in contact with mycelium of strains of opposite mating type, to presumably fertilize the female gametes. This article is protected by copyright. All rights reserved.
... The physiological and cellular mechanisms occurring during sexual reproduction have been described in some model ascomycetes (e.g. Neurospora crassa; Brun et al., 2021), but the interaction with the host plant remains poorly understood for biotrophic and hemibiotrophic plant pathogens. ...
Little is known about the impact of host immunity on sexual reproduction in fungal pathogens. In particular, it is unclear whether crossing requires both sexual partners to infect living plant tissues. We addressed this issue in a three-year experiment investigating different scenarios of Zymoseptoria tritici crosses on wheat according to the virulence (‘vir’) or avirulence (‘avr’) of the parents against a qualitative resistance gene. Co-inoculations (‘vir × vir’, ‘avr × vir’, ‘avr × avr’) and single inoculations were performed on a cultivar carrying the resistance gene (Cellule) and a susceptible cultivar (Apache), in the greenhouse. We assessed the intensity of asexual multiplication by scoring disease severity, and the intensity of sexual reproduction by counting the ascospores discharged from wheat residues. As expected, disease severity was more intense on Cellule for ‘vir × vir’ co-inoculations than for ‘avr × vir’ co-inoculations, with no disease for ‘avr × avr’. However, all types of co-inoculation yielded sexual offspring, whether or not the parental strains caused plant symptoms. Parenthood was confirmed by genotyping (SSR markers), and the occurrence of crosses between (co-)inoculated and exogenous strains (other strains from the experiment, or from far away) was determined. We found that symptomatic asexual infection was not required for a strain to participate in sexual reproduction, and that avirulent strains could be maintained asymptomatically “on” or “in” leaf tissues of plants carrying the corresponding resistant gene for long enough to reproduce sexually. In two of the three years, the intensity of sexual reproduction did not differ significantly between the three types of co-inoculation in Cellule, suggesting that crosses involving avirulent strains are not anecdotal. We discuss the possible mechanisms explaining the maintenance of avirulence in Z. tritici populations and supporting the potential efficacy of cultivar mixtures for limiting resistance gene breakdown.
Highlights
Avirulent Zymoseptoria tritici strains can reproduce sexually in wheat plants carrying the corresponding resistant gene.
Symptomatic infection of plant tissues is not essential for a strain to reproduce sexually.
Avirulent strains can be maintained asymptomatically “on” or “in” leaf tissues of plants carrying the corresponding resistant gene for long enough to reproduce sexually.
Crosses of virulent strains with virulent and avirulent strains in a plant host carrying the corresponding resistance gene can produce offspring with similar population sizes.
Several possible scenarios for sexual crosses can explain the maintenance of avirulence in Zymoseptoria tritici populations evolving in a wheat canopy, particular in cultivar mixtures.
This paper addresses the stability of mycelial growth in fungi and differences between ascomycetes and basidiomycetes. Starting with general evolutionary theories of multicellularity and the role of sex, we then discuss individuality in fungi. Recent research has demonstrated the deleterious consequences of nucleus-level selection in fungal mycelia, favoring cheaters with a nucleus-level benefit during spore formation but a negative effect on mycelium-level fitness. Cheaters appear to generally be loss-of-fusion (LOF) mutants, with a higher propensity to form aerial hyphae developing into asexual spores. Since LOF mutants rely on heterokaryosis with wild-type nuclei, we argue that regular single-spore bottlenecks can efficiently select against such cheater mutants. We then zoom in on ecological differences between ascomycetes being typically fast-growing but short-lived with frequent asexual-spore bottlenecks and basidiomycetes being generally slow-growing but long-lived and usually without asexual-spore bottlenecks. We argue that these life history differences have coevolved with stricter nuclear quality checks in basidiomycetes. Specifically, we propose a new function for clamp connections, structures formed during the sexual stage in ascomycetes and basidiomycetes but during somatic growth only in basidiomycete dikaryons. During dikaryon cell division, the two haploid nuclei temporarily enter a monokaryotic phase, by alternatingly entering a retrograde-growing clamp cell, which subsequently fuses with the subapical cell to recover the dikaryotic cell. We hypothesize that clamp connections act as screening devices for nuclear quality, with both nuclei continuously testing each other for fusion ability, a test that LOF mutants will fail. By linking differences in longevity of the mycelial phase to ecology and stringency of nuclear quality checks, we propose that mycelia have a constant and low lifetime cheating risk, irrespective of their size and longevity.
