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Mating and sexual spore biology
Sexual cycle of P. infestans
The sexual cycle of P. infestans is representative of other heterothallic members of the genus. P. infestans is diploid with A1 and A2 mating types. These represent compatibility types differing in the production and response to mating hormones, rather than dimorphic sexual forms. When an A1 strain detects the A2 hormone, or when an A2 detects the A1 hormone, sexual differentiation begins. The first detectable mating-specific structures are terminal hyphal swellings. These adhere tightly and develop into the male and female gametangia, which are called antheridial and oogonia. The oogonial initial penetrates the antheridial initial and rapidly expands to form the oogonium. Meiosis occurs within both gametangia, yielding haploid nuclei. These fuse to form a diploid nucleus within the developing oospore, which also acquires a multilayered outer wall, nutrient reserves (largely lipids), and other components characterized in notable biochemical and micrographic studies. Days or years later, an oospore will germinate to yield A1 or A2 hyphae.
It is important to consider that mating type is not the same as sex. Each strain of P. infestans (A1 or A2) has the capability to become male or female. However, some strains seem to "prefer" to be male or female. Also, some strains can act male with some partners and female with others!
Mating culture established by growing together an A1 isolate (left) and an A2 (right). Oospores are formed within the agar where the two isolates meet.
See a movie of mating in Phytophthora!
Avi format (medium quality, compressed from my 200 Mb original)
Mov format for Quicktime (medium quality)
Oospores play important roles in disease. These robust structures survive harsh environments to initiate epidemics in subsequent seasons. They are a primarily source of inoculum for epidemics caused by many Phytophthora, downy mildew, and Pythium species. Most P. infestans infections start from asexual spores, but oospores play a significant role. Following nearly a century in which only the A1 mating type was distributed widely, A1 and A2 types are now worldwide. The sexual cycle is thought to occur in regions of potato and tomato production on several continents and the role of overwintering oospores was demonstrated.
The long-term survival of oomycetes is also enhanced by genetic exchanges during mating. Host specificity, aggressiveness, temperature optima, fungicide resistance, and other traits vary between isolates of P. infestans. Recombinants may be more fit or adapted to certain environments. Indeed, some new aggressive strains of P. infestans in the United States are considered to have evolved recently through sexual recombination.
Mating between a transgenic isolate (expressing the GUS gene, which confers the blue color) and wild type strains of P. infestans. Normally, sexual couplings occur between A1 and A2 strains. Panel A, for example, shows a GUS-expressing female (forming the oogonium) mating with the wild type male (forming the antheridium). Panel B shows the opposite interaction. Sometimes, the strains get over-stimulating by mating hormones and mate with themselves (panels C and D).
Questions that we are trying to answer in our studies of germination include:
We have been using several approaches to identify genes and proteins involved in mating and germination. These include the analysis of gene expression using RNA-seq, as well as high-throughput proteomics. We also use the tools of genetics, biochemistry, and cell biology to understand the function of the genes. Some of our current work focuses on genes that may play regulatory roles such as protein kinases, protein phosphatases, and transcription factors.
Mating involving wild type strain and a transformant expressing a fusion between the promoter of a mating-induced gene and the GUS reporter gene. Transcription of the gene is activated early in the pathway, as evidenced by its expression in the young oogonium (left panel).