Intermittent hypoxia in eggs ofAmbystoma maculatum: embryonic development and egg capsule conductance

Abstract

SUMMARY The spotted salamander Ambystoma maculatum breeds in shallow freshwater pools and imbeds its eggs within a common outer jelly matrix that can limit oxygen availability. The eggs are impregnated with the unicellular alga Oophilia amblystomatis, which produces oxygen during the day but consumes oxygen at night. This daily cycle of algal oxygen production drives a diurnal fluctuation of oxygen partial pressure(PO2) within the eggs, the magnitude of which depends on the distance of an egg from the exterior of the jelly matrix and on the ambient PO2 of the pond. We subjected A. maculatum eggs to fluctuating oxygen levels with a variable minimum PO2 and an invariable maximum, to simulate natural conditions, and measured differences in developmental rate,day and stage at hatching, and egg capsule conductance(GO2). Lower minimum PO2 slowed development and resulted in delayed,yet developmentally premature hatching. GO2increased in all treatments throughout development, but PO2 had no detectable effect on the increase. Intermittent hypoxia caused comparable but less pronounced developmental delays than chronic hypoxia and failed to elicit the measurable change in GO2 seen in ambystomatid salamander eggs exposed to chronic hypoxia.

Intracellular invasion of green algae in a salamander host

“…Hutchison and Hammen (14) found no net gain of oxygen from the algae and instead attributed the algal contribution to unknown "growth factors" supplied to the embryo. More refined microelectrode studies have since shown that intracapsular algae do produce a net increase of oxygen (11,12), which confers a meaningful physiological benefit (13). The unexpected intracellular association shown in our study may reveal a benefit to the embryo other than oxygen production.…”

Intracapsular algae provide fixed carbon to developing embryos of the salamanderAmbystoma maculatum

“…Although treatment condition was the primary predictor of embryo length and development throughout the experiment, increased O. amblystomatis abundance alone also had a significant positive effect on both length and development over 30days. These positive effects are likely due to increased oxygen from photosynthesis (Valls and Mills, 2007;Pinder and Friet, 1994) in addition to receiving fixed carbon (discussed below). Eggs maintained in the dark had lower algal populations, and embryos were smaller and less developed.…”

“…A recent discovery revealed that O. amblystomatis is also endosymbiotic; a portion of algal cells penetrates the embryo near the blastopore and settles within tissues and cells of the embryonic salamander . O. amblystomatis benefits A. maculatum embryos, presumably by increasing oxygen within the egg capsules (Valls and Mills, 2007;Pinder and Friet, 1994), which contributes to faster embryonic growth and development, increased survivorship and more synchronous hatching (Tattersall and Spiegelaar, 2008;Gilbert, 1944;Gilbert, 1942). In turn, O. amblystomatis receives embryonic nitrogenous waste (Goff and Stein, 1978) and possibly CO 2 (Hammen, 1962).…”

The effects of atrazine on spotted salamander embryos and their symbiotic alga

“…A similar relationship occurs in several species of amphibians, including at least the salamanders Ambystoma gracile, A. jeffersonianum, A. maculatum, A. tigrinum and the frog Rana sylvatica (Bachmann et al 1986;Gilbert 1942Gilbert , 1944Marco and Blaustein 2000;Pinder and Friet 1994). The alga obtains protection from the egg mass and carbon dioxide and nutrients from the embryos, and the embryos benefit from the oxygen produced by the alga (Bachmann et al 1986;Gilbert 1942Gilbert , 1944Hutchison and Hammen 1958;Marco and Blaustein 2000;Pinder and Friet 1994;Valls and Mills 2007). When oxygen levels drop and carbon dioxide levels rise, which can occur in the absence of the alga, spotted salamander embryos develop more slowly, hatch at an earlier developmental stage, develop deformities, and suffer high mortality (Detwiler and Copenhaver 1940;Mills and Barnhart 1999).…”