Parasexuality in Race 65 Colletotrichum lindemuthianum Isolates

ABSTRACT. Heterokaryosis is the initial step of the parasexual cycle, a process that provides genetic variability in filamentous fungi through the production of heterozygous diploid nuclei. To characterize the parasexual cycle in Colletotrichum lindemuthianum, we evaluated the presence of heterokaryosis, vegetative compatibility reactions, and diploid formation among isolates of Race 65 collected from different Brazilian states. Vegetative compatibility groups were identified among the isolates according to their ability to form heterokaryons. Two heterozygous diploids were selected from compatible heterokaryons, which were characterized by the segregation of the parental auxotrophic markers and by RAPD profiles.     

Some Interactions of Gibberellic Acid with Naringenin (5,7, 4'-trihydroxy flavanone) in the Control of Dormancy and Growth in Plants

The possibility that dormancy and growth are at least partlycontrolled by an interaction between endogenous gibberellinand inhibitor was investigated. Antagonistic effects of twonaturally occurring inhibitors (coumarin and naringenin) upondormancy-breaking or growth-stimulation by gibberellic acid(GA₃) was demonstrated. Naringenin, which occurs in dormantpeach buds, was found competitively to antagonize GAS in thelatter compound's effect of breaking peachbud dormancy. Naringeninwas found to induce a light-requirement for the germinationof ‘Great Lakes’ lettuce seed, which effect couldbe reversed by the addition of GA₃, the extent of the reversalbeing a function of the relative concentrations of the two compounds.Interaction between GA₃ and either coumarin or naringenin innon-dormant tissue (lettuce hypocotyl growth assay) was foundto be non-competitive. An analogy is drawn between the knownoestrogenic and other physiological activities of flavonoids,and the growth-effects of naringenin and hydrangenol.     

Studies in Dormancy of Sycamore I. Seasonal Changes in the Growth-substance Content of the Shoot

The growth substances present in buds and leaves of sycamore(Acer pseudoplatanus) have been studied throughout the annualcycle, in relation to the growth and dormancy of the shoot.Extracts made with 8o per cent. methyl alcohol of the apicalregion and mature leaves were chromatographed and assayed withwheat coleoptile sections, lettuce seeds, cress-roots, and leaf-disksof 8ycamore. An inhibitor was present in all extracts, running at approximatelyRf 0.7 on chrornatograms developed in isopropanol/ammonia. Thiswas found to show various quantitative changes during the courseof the year. Extracts of the apical region showed greatest inhibitingactivity in the early winter, and least when active growth wastaking place in May—June. There appeared to be a gradualdecrease in inhibitor content of the terminal-bud during thecourse of the winter. During late summer and early autumn theinhibitor content of the apex increased. Extracts of the matureleaves made during summer and autumn showed that the inhibitorcontent of the leaves increased until late August, and thendecreased to zero. This latter change is thought to be associatedwith the senescence of the leaves prior to ab8cission. Indirect evidence suggests that the inhibitor is formed in theleaves and transported to the apex, where it accumulates. Theinhibitor was found to inhibit the growth of sycamore tissueitself. Growth-promoting activity was found on most of the chromatograms,and it was shown that this was attributable to sugar ratherthan auxin.     

A native dasyurid predator (common planigale,Planigale maculata) rapidly learns to avoid a toxic invader

Abstract Interactions between invasive species and native fauna afford a unique opportunity to examine interspecific encounters as they first occur, without the complications introduced by coevolution. In northern Australia, the continuing invasion of the highly toxic cane toad Bufo marinus poses a threat to many frog‐eating predators. Can predators learn to distinguish the novel toxic prey item from native prey (and thus, avoid being poisoned), or are longer‐term genetically based changes to attack behaviour needed before predators can coexist with toads? To predict the short‐term impact of cane toads on native predators, we need to know the proportion of individuals that will attack toads, the proportion surviving the encounter, and whether surviving predators learn to avoid toads. We quantified these traits in a dasyurid (common planigale, Planigale maculata) that inhabits tropical floodplains across northern Australia. Although 90% of naïve planigales attacked cane toads, 83% of these animals survived because they either rejected the toad unharmed, or killed and consumed the prey snout‐first (thereby avoiding the toxin‐laden parotoid glands). Most planigales showed one‐trial learning and subsequently refused to attack cane toads for long time periods (up to 28 days). Toad‐exposed planigales also avoided native frogs for up to 9 days, thereby providing an immediate benefit to native anurans. However, the predators gradually learnt to use chemical cues to discriminate between frogs and toads. Collectively, our results suggest that generalist predators can learn to distinguish and avoid novel toxic prey very rapidly – and hence, that small dasyurid predators can rapidly adapt to the cane toad invasion. Indeed, it may be feasible to teach especially vulnerable predators to avoid cane toads before the toads invade, by deploying low‐toxicity baits that stimulate taste‐aversion learning.