Postdoc, University of California Santa Cruz
Bacterial symbiont transmission between hosts: mechanisms, consequences, and causes.
My research focuses on understanding how intracellular symbionts are transmitted from one host generation to the next, and why such transmission strategies have been adopted. All life on Earth, besides some free-living bacteria and archaea, has been impacted by associations with intracellular symbionts. All eukaryotic cells harbor a mitochondrian, and plants also contain chloroplasts, both of which evolved from bacterial ancestors 1.5 billion years ago. New associations have arisen countless times since then by association of free-living bacteria with a host for a range of purposes from parasitic to mutualistic, however research in understanding how these associations establish and persist over evolutionary time is still in its infancy. The mechanism/strategy of transmission likely plays an important role in this process by ensuring that the partners find one another and by linking symbiont and host fitness. Initially, symbionts are thought to start out horizontally transmitted through the environment, similar to how their ancestors established the relationship, but can quickly shift to being inherited. Through studying younger associations, we can both learn about the diversity of symbioses in nature and the principles of the intimate interspecific interactions that formed our own cells.
Abstract: Reliable transmission of symbionts between host generations facilitates the evolution of beneficial and pathogenic associations. Although transmission mode is typically characterized as either vertical or horizontal, the prevalence of intermediate transmission modes, and their impact on symbiont genome evolution, are understudied. Here, we use population genomics to explore mixed transmission modes of chemosynthetic bacterial symbionts in the bivalve Solemya velum. Despite strong evidence for symbiont inheritance through host oocytes, whole-genome analyses revealed signatures of frequent horizontal transmission, including discordant mitochondrial-symbiont genealogies, widespread recombination and a dynamic symbiont genome structure consistent with evolutionary patterns of horizontally transmitted associations. Population-level analyses thus provide a tractable means of ascertaining the fidelity of vertical versus horizontal transmission. Our data support the strong influence horizontal transmission can have on symbiont genome evolution, and shed light on the dynamic evolutionary pressures shaping symbiotic bacterial genomes.The ISME Journal advance online publication, 24 February 2017; doi:10.1038/ismej.2017.10.
Pub.: 25 Feb '17, Pinned: 27 Jun '17
Abstract: Wolbachia is an intracellular endosymbiont present in most arthropod and filarial nematode species. Transmission between hosts is primarily vertical, taking place exclusively through the female germline, though horizontal transmission has also been documented. Several studies indicate that Wolbachia is capable of transfer between somatic and germline cells during nematode development and in adult flies. However, the mechanisms underlying horizontal cell-to-cell transfer remain largely unexplored. Here, we establish a tractable system for probing horizontal transfer of Wolbachia between Drosophila cells in culture using fluorescence in situ hybridization (FISH). First, we show that horizontal transfer is independent of cell-to-cell contact and can efficiently take place through the culture medium within hours. Further, we demonstrate that efficient transfer utilizes host cell phagocytic and clathrin/dynamin-dependent endocytic machinery. Lastly, we provide evidence that this process is conserved between species, showing that horizontal transfer from mosquito to Drosophila cells takes place in a similar fashion. Taken together, our results indicate that Wolbachia utilize host internalization machinery during infection, and this mechanism is conserved across insect species.Our work has broad implications for the control and treatment of tropical diseases. Wolbachia can confer resistance against a variety of human pathogens in mosquito vectors. Elucidating the mechanisms of horizontal transfer will be useful for efforts to more efficiently infect non-natural insect hosts with Wolbachia as a biological control agent. Further, as Wolbachia is essential for the survival of filarial nematodes, understanding horizontal transfer could provide new approaches to treat human infections by targeting Wolbachia Finally, this work provides a key first step toward the genetic manipulation of Wolbachia.
Pub.: 15 Jan '17, Pinned: 27 Jun '17
Abstract: Symbioses between chemoautotrophic bacteria and marine invertebrates are rare examples of living systems that are virtually independent of photosynthetic primary production. These associations have evolved multiple times in marine habitats, such as deep-sea hydrothermal vents and reducing sediments, characterized by steep gradients of oxygen and reduced chemicals. Due to difficulties associated with maintaining these symbioses in the laboratory and culturing the symbiotic bacteria, studies of chemosynthetic symbioses rely heavily on culture independent methods. The symbiosis between the coastal bivalve, Solemya velum, and its intracellular symbiont is a model for chemosynthetic symbioses given its accessibility in intertidal environments and the ability to maintain it under laboratory conditions. To better understand this symbiosis, the genome of the S. velum endosymbiont was sequenced.Relative to the genomes of obligate symbiotic bacteria, which commonly undergo erosion and reduction, the S. velum symbiont genome was large (2.7 Mb), GC-rich (51%), and contained a large number (78) of mobile genetic elements. Comparative genomics identified sets of genes specific to the chemosynthetic lifestyle and necessary to sustain the symbiosis. In addition, a number of inferred metabolic pathways and cellular processes, including heterotrophy, branched electron transport, and motility, suggested that besides the ability to function as an endosymbiont, the bacterium may have the capacity to live outside the host.The physiological dexterity indicated by the genome substantially improves our understanding of the genetic and metabolic capabilities of the S. velum symbiont and the breadth of niches the partners may inhabit during their lifecycle.
Pub.: 25 Oct '14, Pinned: 27 Jun '17