Genetics of Choanephora cucurbitarum

Abstract

Zygospores of Choanephora cucurbitarum (Berkeley et Ravevel) Thaxter germinated by producing germ tubes with terminal germsporangia, germsporangioles on vesicles or germ tubes which continued to grow without producing germsporangia. Zygospores have a light requirement for germination and were stimulated by KMnO4, NaClO and H2O2. Zygospore progeny from crosses between (+) and (-) isolates of g. cucurbitarum consisted of (+), (-) and (+,-) mating types. Mating types of germsporangiospores in each germsporangium were either all (+), or all (-) from respective (+) or (-) zygospore cultures and, all (+,-), or (+) and/or (-) plus (+,-) types from (+,-) zygospore cultures. Mating type heterokaryons (+,-) obtained from zygospore and germsporangiospore cultures in all crosses produced azygospores and zygospores which segregated into (+) and (-) during asexual propagation. All the zygospore and germsporangiospore progenies from the cross between two fast growing isolates were of fast growing type, while all those from the crosses between two slow growing isolates were of slow growing type. The crosses between fast and slow growing isolates gave rise to progeny with both fast and slow growing types. Mating types of zygospore isolates from crosses between two near identical isolates followed 1(+):1(-):2(+,-) ratio. The distribution of mating types (+) and (-), and growth types F and S fit a 1:1 ratio in zygospore progeny in all crosses. Results suggest that mating type as well as growth type are each controlled by a gene with a pair of alleles, and that these two genes are linked to each other with a distance of about 33 cM. The heterokaryotic zygospore progeny from the crosses between F and S types consisted of F(+,-), S(+,-), F,S(+,-), S,F(+,-), F,S(+) and F,S(-). Each of them segregated into two types during sporangiospore formation, suggesting that only two of the four nuclei resulted from meiosis move into the germ tube of a germinating zygospore. Mating type heterokaryons (+,-) formed through fusion of (+) and (-) protoplasts. The (+,-) colonies from protoplast fusion also produced azygospores and zygospores which segregated into (+) and (-) mating types during sporangiospore formation.