Life cycle stages of the amphibian chytrid Batrachochytrium dendrobatidis

Abstract

An overview of the morphology and life cycle of Batrachochytrium dendrobatidis, the cause of chytridiomycosis of amphibians, is presented. We used a range of methods to examine stages of the life cycle in culture and in frog skin, and to assess ultrastructural pathology in the skin of 2 frogs. Methods included light microscopy, transmission electron microscopy with conventional methods as well as high pressure freezing and freeze substitution, and scanning electron microscopy with critical point drying as well as examination of bulk-frozen and freeze-fractured material. Although chytridiomycosis is an emerging disease, B. dendrobatidis has adaptations that suggest it has long been evolved to live within cells in the dynamic tissue of the stratified epidermis. Sporangia developed at a rate that coincided with the maturation of the cell, and fungal discharge tubes usually opened onto the distal surface of epidermal cells of the stratum corneum. A zone of condensed, fibrillar, host cytoplasm surrounded some sporangia. Hyperkeratosis may be due to (1) a hyperplastic response that leads to an increased turnover of epidermal cells, and (2) premature keratinization and death of infected cells.KEY WORDS: Batrachochytrium dendrobatidis · Chytridiomycosis · Fungus · Morphology · Ultrastructure · Transmission and scanning electron microscopy · Pathology · Amphibian Resale or republication not permitted without written consent of the publisherDis Aquat Org 68: [51][52][53][54][55][56][57][58][59][60][61][62][63] 2005 its occurrence on an amphibian host and small subunit ribosomal DNA (ssu-rDNA) sequence of B. dendrobatidis indicate that this fungus is distinctly different from other known chytrids (Berger et al. 1998, James et al. 2000. It is the only member of this phylum to cause disease in a vertebrate.The life cycle of Batrachochytrium dendrobatidis is a simple progression from zoospore to the growing organism, called a thallus, which produces a single zoosporangium (= container for zoospores). The contents of the zoosporangium (also known as a sporangium) cleave into new zoospores which exit the sporangium through one or more papillae. Sexual reproduction has not been observed. Colonial development resulting from the formation of more than 1 sporangium from 1 zoospore is the only known variation of the cycle ). The life cycle seems to be the same in culture and in skin. The duration of the life cycle in vitro is 4 to 5 d at 22°C and is assumed to be the same in amphibian skin, although this has not been tested.Batrachochytrium dendrobatidis discharges zoospores through an inoperculate opening and exhibits monocentric or colonial growth . Longcore et al. (1999) comprehensively described the taxonomic features of B. dendrobatidis and gave detailed morphology based on light microscopic observations of cultures and ultrastructure of serially sectioned zoospores, but did not describe the appearance of life cycle stages using scanning electron microscopy (SEM).Significant ultrastructural differences were not obs...

Host invasion by Batrachochytrium dendrobatidis: fungal and epidermal ultrastructure in model anurans

“…our study design precluded robust statistical comparisons. Our finding that disruption of the epidermal cell maturation cycle was mild and localized in bullfrogs, a low-mortality-risk species for chytridiomycosis, and more severe in wood frogs, a high-mortalityrisk species for chy tri diomycosis (at least at some life stages under laboratory conditions), supports previous work (Daszak et al 2004, Berger et al 2005. replacement (stratum spinosum) cell layers could confer molting plasticity to the bullfrog, allowing this species to derive benefits from abnormal molting without damage to cutaneous function.…”

“…Zoospore cysts and germination tubes were somewhat smaller in Bd than in some other exogenous chytrid species; however, sizes were reasonably similar and, unlike in wetmounted specimens, plane of section could have influenced measurement of Bd features. Infrequent observation of rhizoids in skin corroborates ultrastructure studies of the dainty green tree frog Litoria gracilenta (Berger et al 2005) and White's tree frog L. caerulea .Infected cells of bullfrogs and wood frogs bore a close resemblance to those of dainty green tree frogs with regard to displacement of host organelles and clear zones around zoosporangia (Berger et al 2005). Altered cytoplasm may indicate enzymatic activity Figs.…”

“…The asexual life cycle of Bd consists of a motile zoospore stage and a stationary thallus stage (Berger et al 2005). Zoospores are flagellated and are not bounded by a cell wall.…”

“…On nutrient media, an encysted zoospore develops into a single or colonial (internally divided) zoosporangium (endogenous growth); root-like rhizoids originating from 1 or more points on the zoosporangium may be numerous . In an amphibian host, cellular contents of a superficially encysted zoospore leave the cyst and enter a host cell (exogenous growth), inside which a single or colonial zoosporangium develops; rhizoids are usually less abundant than on nutrient media (Berger et al 2005). The specifics of amphibian host invasion by Bd (i.e.…”

“…Branching rhizoids may originate from the tube or walls of zoosporangia (Barr & Désaulniers 1987, Longcore 1995. Maturation of an endobiotic Bd zoosporangium culminates in cleaving of zoosporangial contents into zoospores, formation of 1 or more zoosporedischarge papillae leading to the surface of the host or into intercellular spaces, and release of zoospores (Berger et al 2005). Bd is the only chytrid known to parasitize vertebrate hosts and is one of few endobiotic members of order Rhizophydiales (Letcher et al 2006).…”