Nuclei are mobile processors enabling specialization in a gigantic single-celled syncytium

Abstract

In multicellular organisms, the specification, coordination, and compartmentalization of cell types enable the formation of complex body plans. However, some eukaryotic protists such as slime molds generate diverse and complex structures while remaining in a multinucleated syncytial state. It is unknown if different regions of these giant syncytial cells have distinct transcriptional responses to environmental encounters, and if nuclei within the cell diversify into heterogeneous states. Here we performed spatial transcriptome analysis of the slime mold Physarum polycephalum in the plasmodium state under different environmental conditions, and used single-nucleus RNA-sequencing to dissect gene expression heterogeneity among nuclei. Our data identifies transcriptome regionality in the organism that associates with proliferation, syncytial substructures, and localized environmental conditions. Further, we find that nuclei are heterogenous in their transcriptional profile, and may process local signals within the plasmodium to coordinate cell growth, metabolism, and reproduction. To understand how nuclei variation within the syncytium compares to heterogeneity in single-nucleated cells, we analyzed states in single Physarum amoebal cells. We observed amoebal cell states at different stages of mitosis and meiosis, and identified cytokinetic features that are specific to nuclei divisions within the syncytium. Notably, we do not find evidence for predefined transcriptomic states in the amoebae that are observed in the syncytium. Our data shows that a single-celled slime mold can control its gene expression in a region-specific manner while lacking cellular compartmentalization, and suggests that nuclei are mobile processors facilitating local specialized functions. More broadly, slime molds offer the extraordinary opportunity to explore how organisms can evolve regulatory mechanisms to divide labor, specialize, balance competition with cooperation, and perform other foundational principles that govern the logic of life.

Microtubules in Microorganisms: How Tubulin Isotypes Contribute to Diverse Cytoskeletal Functions

“…Chlamydomonas reinhardtii tua1, tua2 tub1, tub2 each isoform: differential anti-tubulin drug resistance Kato- Minoura et al (2020). Physarum polycephalum altA, altB(N), altB(E), betA, betB, betC altA, altB(N), altB(E): MT assembly in plasmodia and myxamoeba Burland et al (1983), Roobol et al (1984), Cunningham et al (1993);Gerber et al (2022), altA builds flagellar MTs via PTM in amoeba and flagellates Green and Dove (1984), Blindt et al (1986), Diggins and Dove (1987), Sasse et al (1987); betA: MT structure assembly during myxamoeba stage Burland et al (1988), Diggins-Gilicinski et al (1989), Gerber et al (2022; betB: constitutively expressed in plasmodia and myxamoeba Werenskiold et al (1988), Paul et al (1989), benzimidazole resistance to both plasmodia and myxamoeba Burland et al (1984); betC: anastral mitotic spindle assembly in plasmodia Burland et al (1983), Roobol et al (1984), Gerber et al (2022), MT assembly in developing cells during amoeboid-plasmodial transition Solnica-Krezel et al (1988). *In Table 2, gene names are presented using organism-specific syntax.…”

“…Expression of the α-tubulin isotypes is differentially regulated in various life stages of Physarum. Despite having only 1 MT organelle, the anastral mitotic spindle, in the coenocytic plasmodia stage, more tubulin isotypes were found to be definitively expressed in this stage [ALTA, ALTB(N), ALTB(E), BETB, BETC] than the amoeba-like myxamoeba stage (ALTA, BETA, BETB) (Burland et al, 1983;Roobol et al, 1984;Gerber et al, 2022). Although the α-tubulin population contains two isoelectric variants, one was found to originate via acetylation of AltA (Green and Dove, 1984;Blindt et al, 1986;Diggins and Dove, 1987;Sasse et al, 1987), leaving only one major α-isotype expressed in the myxamoeba phase (Singhofer-Wowra and Little, 1987).…”

“…Of the three β-tubulin isotypes, BETA and BETC are exclusively expressed and incorporated into MT structures of the myxamoeba and plasmodia stages, respectively (Burland et al, 1988;Diggins-Gilicinski et al, 1989;Gerber et al, 2022). On the other hand, BETB is constitutively expressed during both the myxamoeba and plasmodia phases (Werenskiold et al, 1988) and incorporated into all MT structures (Paul et al, 1989).…”