Genetic Resources (Including Wild and Cultivated Solanum Species) and Progress in their Utilisation in Potato Breeding

The genetic resources available for the improvement of the cultivated potato (Solanum tuberosum) are reviewed along with progress in their utilisation. The conclusions are as follows. The wild and cultivated species of potato have been utilised in potato breeding to good effect, but only a very small sample of the available biodiversity has been exploited. New knowledge and technology will open possibilities for much greater use of these genetic resources in breeding. The strategy for utilising the cultivars native to Latin America will either be the introgression of desirable genes or the direct use of parents from improved populations, depending on how far modern S. tuberosum cultivars have genetically diverged from them and the extent to which S. tuberosum cultivars have been improved in the process. Molecular marker-assisted selection will be used for faster introgression of desirable genes from wild species, and the possibility exists of moving genes directly from wild species to cultivated potato with transgenic methods. New cultivars will continue to come from crosses between pairs of parents with complementary features but adapted to local growing conditions. However, increasingly these parents will possess desirable genes which have been introgressed from wild species and may also be from complementary groups of cultivated germplasm to exploit hybrid vigour. Successful cultivars may be genetically modified, if consumers see benefits in the use of the technology, to introduce genes not present in cultivated potatoes and their wild relatives to achieve novel biochemistry and further desirable improvements.     

The transmission potential of Loma salmonae (Microspora) in the rainbow trout, Oncorhynchus mykiss (Walbaum), is dependent upon the method and timing of exposure

The ability of a parasite to transmit from one fish to another is important in the dissemination of disease. Groups of 25 naive rainbow trout (RBT), Oncorhynchus mykiss (Walbaum), were exposed to Loma salmonae by feeding on the viscera (gills, hearts and spleens) from L. salmonae -infected donor RBT (DRBT) or by cohabitation with infected DRBT. Exposure occurred 3, 7, 11 and 15 weeks after the DRBT were infected. All naive RBT were examined 7 weeks post-exposure (PE) to the DRBT. Naive RBT, exposed to DRBT at week 3 PE, by feeding on viscera or by cohabitation, failed to develop visible branchial xenomas. Cohabiting naive RBT with DRBT, at week 7 PE and week 11 PE, resulted in the development of branchial xenomas. Xenomas failed to develop in naive RBT exposed via cohabitation to week 15 PE DRBT. Naive RBT, exposed by feeding on the viscera of DRBT at week 7 PE, week 11 PE and week 15 PE, developed branchial xenomas. The transmission potential of viscera from L. salmonae -infected DRBT at week 15 and week 20 PE was also examined. Naive RBT, fed with viscera free of visible branchial xenomas, from DRBT at week 15 PE and week 20 PE, developed branchial xenomas by week 7 PE. A polymerase chain reaction (PCR) was used to detect L. salmonae DNA from the water and sediments of a tank of L. salmonae -infected RBT at week 7 PE. The method and timing of exposure of naive fish to L. salmonae -infected fish are important in disease transmission and may be useful in predicting and preventing disease outbreaks in aquaculture.     

Loma salmonae-associated xenoma onset and clearance in rainbow trout, Oncorhynchus mykiss (Walbaum): comparisons of per os and cohabitation exposure using survival analysis

Survival analysis techniques were used to compare experimental exposure methods of Loma salmonae (Microspora) in rainbow trout (RBT) by measuring xenoma onset and clearance time. Twenty‐eight naive RBT were exposed per os (fed L. salmonae spores) and 28 RBT were exposed by cohabitation with 28 L. salmonae‐infected RBT. Exposed fish were examined once every week (7 days) post exposure (PE). For xenoma onset, the median survival time, the time to first appearance of branchial xenomas, was 6 weeks PE for both per os and cohabitation exposure. For xenoma clearance, the median survival time, the time to total clearance of branchial xenomas, was 10 weeks for per os exposure and 12 weeks for cohabitation exposure. There was a significant difference between the survival curves of per os fish compared with cohabited fish for both xenoma onset and xenoma clearance. The incidence rate of xenoma development was greater for per os exposure (0.1745 cases per fish‐week) compared with cohabitation exposure (0.1342 cases per fish‐week). Similarly, the rate of xenoma clearance was greater for per os exposure (0.1028 cases per fish‐week) compared with cohabitation exposure (0.0885 cases per fish‐week). Differences between exposure methods were attributed to differences in the frequency and duration of exposure to spores. Survival analysis was useful for examining the onset and clearance of L. salmonae and may have applications for understanding disease dynamics in aquaculture.     

New sources of resistance to the potato cyst nematodes Globodera pallida and G. rostochiensis in the Commonwealth Potato Collection

The Commonwealth Potato Collection contains 77 species following the incorporation of the personal collection of Professor JG Hawkes. The material originates from several expeditions to the Americas and represents a potentially valuable source of resistance to pests and pathogens, including potato cyst nematodes (PCN). The germplasm from the Hawkes collection screened in this study included taxa mainly from Mexico, Bolivia, Argentina and Peru. Seedlings of 198 accessions, from 63 different species were screened for reaction to the PCN species G. pallida and G. rostochiensis. Of the 198 accessions tested, 56% contained G. pallida-resistant clones and 53% contained clones resistant to G. rostochiensis. Twelve species in this survey are believed to contain novel, previously unreported sources of resistance. The geographical distribution of resistance indicated a contrast between the two PCN species. Although, as in previous studies, resistance to G. rostochiensis was centred on Bolivia and Argentina, G. pallida resistance was found across the geographical range of tuber-bearing Solanum. The possibility of cross-resistance arising from selective pressure from the more northern species G. mexicana is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Musculoskeletal responses of 2-year-old Thoroughbred horses to early training. 4. Morphometric,microscopic and biomechanical properties of the digital tendons of the forelimb

AIM: To determine the weight, volume, density and cross-sectional area (CSA) of the digital flexor tendons, common digital extensor tendon (CDET) and suspensory ligament (SL) of the forelimb of young Thoroughbred horses in early training, and to assess the response to a training programme of known parameters of superficial digital flexor tendon (SDFT) tissue at mid-metacarpal level.

METHODS: The tendons of seven 2-year-old Thoroughbred horses in training were inspected, transected into segments of known length, and compared with those from seven untrained horses matched for age, sex and breed. The weight, volume, density and CSA of each segment, and the crimp angle, histological features, and biomechanical ultimate stress and stiffness of tendon samples from the mid-metacarpal region of the SDFT were determined.

RESULTS: There was no macroscopic evidence of swelling or discolouration in any of the tendon segments or cut surfaces. The volumes of SDFT and CDET segments of horses in the trained group were significantly greater than those in the untrained group (p=0.036 and p=0.039, respectively). A greater increase in volume than weight resulted in a lower density in the SDFT but not CDET in trained compared with untrained horses (p=0.038). CSA of these two tendons was significantly greater in the trained group (p=0.002 and 0.036, respectively), the percentage increase being greater in the CDET than the SDFT. The number of tenocytes at four sites in the mid-metacarpal SDFT region was less in trained than untrained horses (p=0.025). There was no histological evidence of inflammation, and no difference in crimp angle between groups. There was no significant between-group difference in stiffness or ultimate stress of tendon strips.

CONCLUSIONS: Volume and CSA of the SDFT and CDET were larger in trained than untrained horses. The SDFT was less dense in the trained group. Because no evidence of tendonitis was detected and training appeared to have no significant effect on crimp angle or biomechanical properties of tendon strips, the size and density changes were presumed to be adaptive and induced by the training.

CLINICAL RELEVANCE: Although evident in this in vitro study, the detection of adaptive from initial pathological increase in size of the SDFT is likely to be difficult in vivo.     

Degenerative spinal disease in large felids.

Degenerative spinal disorders, including intervertebral disc disease and spondylosis, seldom occur in domestic cats. In contrast, a retrospective study of 13 lions (Panthera leo), 16 tigers (Panthera tigris), 4 leopards (Panthera pardis), 1 snow leopard (Panthera uncia), and 3 jaguars (Panthera onca) from the Knoxville Zoo that died or were euthanatized from 1976 to 1996 indicated that degenerative spinal disease is an important problem in large nondomestic felids. The medical record, radiographic data, and the necropsy report of each animal were examined for evidence of intervertebral disc disease or spondylosis. Eight (three lions, four tigers, and one leopard) animals were diagnosed with degenerative spinal disease. Clinical signs included progressively decreased activity, moderate to severe rear limb muscle atrophy, chronic intermittent rear limb paresis, and ataxia. The age at onset of clinical signs was 10-19 yr (median = 18 yr). Radiographic evaluation of the spinal column was useful in assessing the severity of spinal lesions, and results were correlated with necropsy findings. Lesions were frequently multifocal, included intervertebral disc mineralization or herniation with collapsed intervertebral disc spaces, and were most common in the lumbar area but also involved cervical and thoracic vertebrae. Marked spondylosis was present in the cats with intervertebral disc disease, presumably subsequent to vertebral instability. Six of the animals' spinal cords were examined histologically, and five had acute or chronic damage to the spinal cord secondary to disc protrusion. Spinal disease should be suspected in geriatric large felids with decreased appetite or activity. Radiographic evaluation of the spinal column is the most useful method to assess the type and severity of spinal lesions.     

Evidence for new sources of NOX in the lower atmosphere

Laboratory studies show that the reaction of short-lived O2(B3Sigmau) molecules (lifetime approximately 10 picoseconds) with N2 and the photodissociation of the N2:O2 dimer produce NOx in the stratosphere at a rate comparable to the oxidation of N2O by O(1D). This finding implies the existence of unidentified NOX sinks in the stratosphere. The NO2 observed in this experiment is isotopically heavy with a large 15N/14N enhancement. However, photodissociation of this NO2 unexpectedly produced NO molecules with a low 15N/14N ratio. The diurnal odd-nitrogen cycle in the stratosphere will be marked by a complex isotope signature that will be imprinted on the halogen and HOX catalytic cycles.