# Spiroplasma

Spiroplasma is a genus of Mollicutes, a group of small bacteria without cell walls. Spiroplasma shares the simple metabolism, parasitic lifestyle, fried-egg colony morphology and small genome of other Mollicutes, but has a distinctive helical morphology, unlike Mycoplasma. It has a spiral shape and moves in a corkscrew motion. Many Spiroplasma are found either in the gut or haemolymph of insects where they can act to manipulate host reproduction, or defend the host as endosymbionts. Spiroplasma are also disease-causing agents in the phloem of plants. Spiroplasmas are fastidious organisms, which require a rich culture medium. Typically they grow well at 30 °C, but not at 37 °C. A few species, notably Spiroplasma mirum, grow well at 37 °C (human body temperature), and cause cataracts and neurological damage in suckling mice. The best studied species of spiroplasmas are Spiroplasma poulsonii, a reproductive manipulator and defensive insect symbiont, Spiroplasma citri, the causative agent of citrus stubborn disease, and Spiroplasma kunkelii, the causative agent of corn stunt disease.

## Human pathogenicity

There is some disputed evidence for the role of spiroplasmas in the etiology of transmissible spongiform encephalopathies (TSEs), due primarily to the work of Frank Bastian, summarized below. Other researchers have failed to replicate this work, while the prion model for TSEs has gained very wide acceptance. A 2006 study appears to refute the role of spiroplasmas in the best small animal scrapie model (hamsters). Bastian et al. (2007) have responded to this challenge with the isolation of a spiroplasma species from scrapie-infected tissue, grown it in cell-free culture, and demonstrated its infectivity in ruminants.

## Insect symbioses

Many Spiroplasma strains are vertically transmitted endosymbionts of Drosophila species, with a variety of host-altering mechanisms similar to Wolbachia. These strains are from the Spiroplasma poulsonii clade, and can have important effects on host fitness. The S. poulsonii strain of Drosophila neotestacea protects its host against parasitic nematodes. This interaction is an example of defensive symbiosis, where the fitness of the symbiont is intricately tied to the fitness of the host. The D. neotestacea S. poulsonii also defends its fly host from infestation by parasitic wasps. The mechanism through which S. poulsonii attacks nematodes and parasitic wasps relies on the presence of toxins called ribosome-inactivating proteins (RIPs), similar to Sarcin or Ricin. These toxins depurinate a conserved adenine site in eukaryotic 28S ribosomal RNA called the Sarcin-Ricin loop by cleaving the N-glycosidic bond between the rRNA backbone and the adenine. Spiroplasma associations highlight a growing movement to consider heritable symbionts as important drivers in patterns of evolution. Protection against wasp attack can be thermally sensitive, ablated at lower environmental temperatures.
The S. poulsonii strain of Drosophila melanogaster can also attack parasitoid wasps, but is not regarded as a primarily defensive symbiont. This is because this D. melanogaster Spiroplasma (called MSRO) kills D. melanogaster eggs fertilized by Y-bearing sperm. This mode of reproductive manipulation benefits the symbiont as the female fly has a greater reproductive output than males. Work by Veneti and colleagues demonstrated that male-killing was ablated by loss of function of any gene in the dosage compensation complex (DCC), leading to the hypothesis that the target of male-killing was the single X chromosome of males, and enabled by the DCC binding to this chromosome. Work in D. nebulosa demonstrated male death was associated with widespread apoptosis in male embryos during mid/late embryogenesis. The genetic basis of this male-killing was discovered in 2018, solving a decades-old mystery of how the bacteria targeted male-specific cells. In an interview with the Global Health Institute, Dr. Toshiyuki Harumoto said this discovery is the first example of a bacterial effector protein that affects host cellular machinery in a sex-specific manner, and the first endosymbiont factor identified to explain the cause of male-killing. Thus it should have a big impact on the fields of symbiosis, sex determination, and evolution.
Beyond Drosophila, Spiroplasma of the ixodetis, apis, chrysopicola, citri, mirum, and poulsonii clades are found in many insects and arthropods, including ticks, spiders, bees, ants, beetles, and butterflies. Male-killing is also found in the Spiroplasma of the ladybird beetles Adalia bipunctata and Harmonia axyridis, the plain tiger butterfly, the lacewing Mallada desjadinisi, and the pea aphid Acyrthosiphum pisum. In the plain tiger butterfly, the consequences have led to speciation.

## Plant diseases

Spiroplasma citri is the causative agent of Citrus stubborn disease, a plant disease affecting species in the genus Citrus. It infects the phloem of the affected plant, causing fruit deformities. Spiroplasma kunkelii is also referred to as Corn Stunt Spiroplasma as it is the causative agent of Corn stunt disease, a disease of corn and other grasses that stunts plant growth. Spiroplasma kunkelii represents a major economic risk, as corn production in the United States is an industry worth over $50 billion. Both Spiroplasma citri and Spiroplasma kunkelii are transmitted by leafhoppers.

## Genetics and Molecular evolution

Spiroplasma, like other mollicutes, have a distinct genetic code, with two rather than three stop codons. Molecular evolution studies, using Spiroplasma passaged vertically in Drosophila, indicate a very fast rate of molecular evolution. Spiroplasma genomes are commonly extremely AT rich, can contain a variety of prophage (viral) elements, and also plasmids.CRISPR defences are found in some members of the genus. Genome sizes are generally between 0.7 and 2.2 Mb.

## Phylogeny

The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN) and National Center for Biotechnology Information (NCBI).
| 16S rRNA based LTP_10_2024 | 120 marker proteins based GTDB 09-RS220 |
| Metamycoplasmataceae Mycoplasmoidales Mycoplasmoidaceae Spiroplasma~1 Spiroplasma ixodetis Tully et al. 1995 Spiroplasma platyhelix Williamson et al. 1997 Mycoplasmatales Spiroplasma S. eriocheiris Wang et al. 2011 S. atrichopogonis Koerber et al. 2005 S. mira corrig. Tully et al. 1982 S. chrysopicola Whitcomb et al. 1997 S. syrphidicola Whitcomb et al. 1996 S. penaei Nunan et al. 2005 S. insolita corrig. Hackett et al. 1993 S. leucomae Oduori, Lipa & Gasparich 2005 S. poulsonii Williamson et al. 1999 S. phoenicea corrig. Saillard et al. 1987 S. kunkelii Whitcomb et al. 1986 S. citri Saglio et al. 1973 S. mellifera corrig. Clark et al. 1985 Spiroplasmataceae Spiroplasma Spiroplasma alleghenense Adams et al. 1997 Spiroplasma sabaudiense Abalain-Colloc et al. 1987 Spiroplasma lampyridicola Stevens et al. 1997 Spiroplasma leptinotarsae Hackett et al. 1996 Spiroplasma clarkii Whitcomb et al. 1993 Spiroplasma apis Mouches et al. 1984 Spiroplasma montanense Whitcomb et al. 1997 Spiroplasma taiwanense Abalain-Colloc et al. 1988 Spiroplasma monobiae Whitcomb et al. 1993 Spiroplasma cantharicola Whitcomb et al. 1993 Spiroplasma diminutum corrig. Williamson et al. 1996 Spiroplasma floricola Davis, Lee & Worley 1981 Spiroplasma diabroticae Carle et al. 1997 Mesoplasma melaleucae (Tully et al. 1990) Gupta, Son & Oren 2019 Spiroplasma culicicola Spiroplasma chinense Spiroplasma velocicrescens Spiroplasma litorale Spiroplasma corruscae Spiroplasma turonicum Spiroplasma helicoides Spiroplasma gladiatoris Spiroplasma lineolae Spiroplasma tabanidicola species‑group 2 Mycoplasmataceae "Edwardiiplasma" Williamsoniiplasma "Tullyiplasma" Entomoplasma Mesoplasma Mycoplasma /Entomoplasmataceae /Entomoplasmatales | "Ca. Spiroplasma holothuricola" He et al. 2018 Mycoplasmoidaceae "Hepatoplasmataceae" Metamycoplasmataceae VBWQ01 Spiroplasma_D Spiroplasma ixodetis Spiroplasma platyhelix Mycoplasmataceae Spiroplasma S. eriocheiris S. mira S. chrysopicola S. syrphidicola S. poulsonii S. phoenicea S. citri S. mellifera Spiroplasma_B Spiroplasma alleghenense Spiroplasma sabaudiense Spiroplasma_A Spiroplasma litorale Spiroplasma corruscae Spiroplasma turonica Spiroplasma taiwanense Spiroplasma floricola Spiroplasma cantharicola Spiroplasma diminuta Spiroplasma monobiae Spiroplasma apis Spiroplasma clarkii Spiroplasma chinense Spiroplasma culicicola Spiroplasma helicoides Spiroplasma gladiatoris Spiroplasma tabanidicola "Edwardiiplasma" Williamsoniiplasma "Tullyiplasma" Entomoplasma Mesoplasma Mycoplasma |