The Inductive Role of Bacterial Symbionts in the Morphogenesis of a Squid Light Organ

SYNOPSIS. The association of the sepiolid squid Euprymna scolopeswith its marine luminous bacterial symbiont Vibrio fischeriis an emerging model system to study the initiation and developmentof bacterial symbioses in higher animals, in particular theinfluence of bacteria on the ontogenic development of symbiotic-specifichost tissues. Experiments comparing the development of juvenilesquid infected with symbiotic V. fischeri with that of uninfectedjuveniles suggest postembryonic development of the light organrequires cell-cell interactions with the bacterial symbionts.The presence of symbiotic bacteria induces specific morphologicalchanges by affecting such fundamental processes as cell deathand cell differentiation. The surface of the juvenile organis largely composed of ciliated cells that appear to facilitateinfection of the light organ. These cells begin to undergo celldeath within hours of infection with symbiotic V. fischeri.Within three days the epithelial cells that form the bacteriacontainingcrypts of the light organ increase in size; these cells do notappear mitotically active, and may represent a terminally differentiatedstate. The light organs of uninfected juvenile E. scolopes,however, do not exhibit any of these early postembryonic developmentalevents but remain in a state of arrested morphogenesis.     

The Development of Cooperative Associations Between Animals and Bacteria: Establishing Detente Among Domains

SYNOPSIS. détente = n.(fr.) the relaxation of tensionsbetween two nations, usually through cooperation and negotiation.[The Random House Dictionary, 2nd Edition, 1987] Despite the ubiquitous occurrence of cooperative associationsbetween animals and bacteria, there is little understandingof how these interactions arose, how they evolved, and how theypersist. Thus, an extensive database concerning the influenceof bacteria on developmental pathways is not yet available.However, in much the same way that mutually beneficial liaisonsare created between nations with vastly different historiesand cultures, it is likely that highly refined developmentalmechanisms exist in which a type of detente is created to retainthe integrity of the associations between the partners, bothwithin and between generations. These developmental pathwayswould be responsible for insuring that a balance of cell growthis established and maintained among the community members, comprisedof animal and microbial cells, such that neither form of pathogenesis,i.e., overgrowth (war) or aposymbiosis (isolation), ensues.This contribution examines aspects of how alliances with prokaryotesmay have been integrated into the mechanisms and patterns ofhost animal developmental programs.     

Animal-Bacterial Interactions in the Early Life History of Marine Invertebrates: The Euprymna scolopes/Vibrio fischeri Symbiosis

SYNOPSIS. The symbiotic association between the Hawaiian sepiolidsquid Euprymna scolopes and the marine luminous bacterium Vibriofischeri is being developed as a model system for the studyof animal-bacterial interactions during development. Changesin light organ morphology during embryogenesis foster successfulinfection of the light organ with the proper bacterial partner.These embryonic events of light organ morphogenesis includethe elaboration of an epithelial surface with a complex ciliated,microvillous field. The squid host hatches without the bacterialsymbionts, but acquires them within hours from the free-livingpopulation of the bacteria in the water column. Upon exposureto the proper symbionts, the host organ undergoes a series ofmorphogenetic changes, including loss of the ciliated, microvillousfield. The light organ then goes on to mature into a morphologicalconfiguration that serves to promotethe maintenance of a stableassociation with the bacteria and that correlates with the useof the bacterial bioluminescence in behavior of the host. Thissymbiosis is discussed in light of other cyclically transmittedanimal-bacterial associations.     

Crypsis in the Pelagic Environment

Pelagic aquatic environments differ from terrestrial environmentsin being three-dimensional and relatively homogeneous, ratherthan two-dimensional and heterogeneous. The present paper examinesthe causes and consequences of these differences in the contextof their influence on the interactions of animals with environmentallight. Particular emphasis is placed on light as a determinantof effective modes of crypsis in the two different habitats.The terrestrial world has selected for the expression of crypticityin the form of superficial color patterns. The heterogeneityof this habitat has resulted in evolutionary divergence of thesesuperficial color patterns, often in very closely-related animals.In contrast, in the homogeneous pelagic aquatic habitats, evolutionaryconvergence on three main forms of crypsis is evident: (1) transparency;(2) reflection of most, if not all visible wavelengths; and,(3) ventral bioluminescence as counterillumination; thus, tobe cryptic most animals in these habitats use one or a combinationof these modalities to variously transmit, reflect or mimicenvironmental light. In the present paper, special attentionis given to transparency as the most prevalent, yet least understood,of these mechanisms that are used in predator-prey interactions.