A region of maize chromosome 2 affects response to downy mildew pathogens.

Quantitative trait loci (QTLs) for downy mildew resistance in maize were identified based on co-segregation with linked restriction fragment length polymorphisms or simple sequence repeats in 220 F2 progeny from a cross between susceptible and resistant parents. Disease response was assessed on F3 families in nurseries in Egypt, Thailand, and South Texas and after inoculation in a controlled greenhouse test. Heritability of the disease reaction was high (around 93% in Thailand). One hundred and thirty polymorphic markers were assigned to the ten chromosomes of maize with LOD scores exceeding 4.9 and covering about 1,265 cM with an average interval length between markers of 9.5 cM. About 90% of the genome is located within 10 cM of the nearest marker. Three putative QTLs were detected in association with resistance to downy mildew in different environments using composite interval mapping. Despite environmental and symptom differences, one locus on chromosome 2 had a major effect and explained up to 70% of the phenotypic variation in Thailand where disease pressure was the highest. The other two QTLs on chromosome 3 and chromosome 9 had minor effects; each explained no more than 4% of the phenotypic variation. The three QTLs appeared to have additive effects on resistance, identifying one major gene and two minor genes that contribute to downy mildew resistance.

Use of SSRs for establishing heterotic groups in subtropical maize.

Heterotic groups and patterns are of fundamental importance in hybrid breeding. The objectives of our research were to: (1) investigate the relationship of simple sequence repeats (SSR) based genetic distances between populations and panmictic midparent heterosis (PMPH) in a broad range of CIMMYT maize germplasm, (2) evaluate the usefulness of SSR markers for defining heterotic groups and patterns in subtropical germplasm, and (3) examine applications of SSR markers for broadening heterotic groups by systematic introgression of other germplasm. Published data of two diallels and one factorial evaluated for grain yield were re-analyzed to calculate the PMPH in population hybrids. Additionally, 20 pools and populations widely used in CIMMYT's breeding program were assayed with 83 SSR markers covering the entire maize genome. Correlations of squared modified Roger's distance (MRD(2)) and PMPH were mostly positive and significant, but adaption problems caused deviations in some cases. For intermediate- and early-maturity subtropical germplasm, two heterotic groups could be suggested consisting of a flint and dent composite. We concluded that the relationships between the populations obtained by SSR analyses are in excellent agreement with pedigree information. SSR markers are a valuable complementation to field trials for identifying heterotic groups and can be used to introgress exotic germplasm systematically.

Identification of QTLs conferring resistance to downy mildews of maize in Asia.

Downy mildew is one of the most destructive diseases of maize in subtropical and tropical regions in Asia. As a prerequisite for improving downy mildew resistance in maize, we analyzed quantitative trait loci (QTLs) involved in resistance to the important downy mildew pathogens--Peronosclerospora sorghi (sorghum downy mildew) and P. heteropogoni (Rajasthan downy mildew) in India, P. maydis (Java downy mildew) in Indonesia, P. zeae in Thailand and P. philippinensis in the Philippines--using a recombinant inbred line population derived from a cross between Ki3 (downy mildew resistant) and CML139 (susceptible). Resistance was evaluated as percentage disease incidence in replicated field trials at five downy mildew 'hotspots' in the four countries. Heritability estimates of individual environments ranged from 0.58 to 0.75 with an across environment heritability of 0.50. Composite interval mapping was applied for QTL detection using a previously constructed restriction fragment length polymorphism linkage map. The investigation resulted in the identification of six genomic regions on chromosomes 1, 2, 6, 7 and 10 involved in the resistance to the downy mildews under study, explaining, in total, 26-57% of the phenotypic variance for disease response. Most QTL alleles conferring resistance to the downy mildews were from Ki3. All QTLs showed significant QTL x environment interactions, suggesting that the expression of the QTL may be environment-dependent. A strong QTL on chromosome 6 was stable across environments, significantly affecting disease resistance at the five locations in four Asian countries. Simple-sequence repeat markers tightly linked to this QTL were identified for potential use in marker-assisted selection.

Response of the ovary in the catfish, Heteropneustes fossilis (Bloch), to various combinations of photoperiod and temperature.

Responses of the ovary of the catfish, Heteropneustes fossilis, to various regimes of photoperiod and temperature, were studied during the different periods of the annual ovarian cycle. During the preparatory and post-spawning periods moderate temperatures of 25 degrees C and above, regardless of photoperiod, were more favorable for the formation of yolky (stage III) oocytes than coller temperatures (below 23 degrees C). Even under conditions of continuous dark or light, ovarian development occurred as a function of warm temperatures. Gravid ovaries were maintained beyond the spawning period only with a temperature of 30 degrees C, photoperiod notwithstanding. Nevertheless, moderate or cool temperatures did not prevent the ovarian regression that occurred by the first week of October in the post-spawning period. Regulation of ovarian activity at least in part by an endogenous circannual rhythm appears possible.

Responses of the regressed ovary of the catfish, Heteropneustes fossils (Bloch), to interrupted-night photoperiods.

Interrupted-night photoperiod schedules were used to explore the possibility of the involvement of a ciradian rhythm in photosensitivity in ovarian recrudescence in the catfish, Heteropneustes fossilis. Interruption of the night of a basic short day (L/D 6:18) treatment by 1-h of light between 00(00) and 01(00) induced ovarian recrudescence which was significantly higher than that in the control group exposed to L/D 7:17. Scanning of the major part of the dark period (20(00-05(00)) to determine the duration of the photo-inducible phase shows that the photo-inducible phase lies between 20(00) and 03(00) with a peak from 00(00) to 01(00). Even 1-h of light in a regimen of L/D 1:23 presented between 20(00)-21(00), 22(00)-23(00), 00(00)-01(00), 02(00)-03(00), or 04(00)-05(00), produced significant ovarian recrudescence with a peak between 22(00) and 23(00). These results provide a clear demonstration of the possible involvement of a circadian mechanism for photoperiod measurement in the catfish.