Genetic control of leucine aminopeptidase and esterase isozymes in the interspecific cross Cucurbita ecuadorensis x C. maxima

Starch gel electrophoretic analyses of crude seed extracts of Cucurbita ecuadorensis, C. maxima, their F₁ and F₂, and three of the four possible interspecific backcrosses reveal that the genus is polymorphic for alpha-naphthyl acetate esterases (Est) and leucine aminopeptidase (LAP). The two electrophoretic forms of both Est and LAP are controlled by codominant alleles. The two loci do not exhibit linkage. Neither the LAP nor the Est phenotypes exhibit a significant deviation from the expected 1:1 ratio in interspecific backcrosses when the donor parent alleles are transmitted through female gametes, but there is a significant deviation for Est when transmission is through male gametes. Differential gametic selection involving the Est-1 locus suggests structural differences between the genomes of the parental species for the chromosomal region in which this locus occurs. No structural differences are indicated between the parental genomes for the chromosome region bearing the Lap-1 locus.     

The segregation pattern of a translocation quadrivalent

The segregation pattern of a translocation quadrivalent was studied in three hybrid families of the tetraploid (2n=28) oat, of the Avena strigosa polyploid complex. The rate of IV formation in F₁ was high and the fertility was normal. The adjacent-alternate orientation of this quadrivalent was very variable. Conspicuous variation in this frequency was found between anthers of the same floret and between florets. The alternate type was usually more common toward the end of MI. The adjacent-alternate ratio was found to be an unreliable measure for calculating the type of gametes produced by the F₁ and the F₂ plants derived from them. An attempt was made to determine the type of viable gametes produced on the F₁ and their frequency by examining the cytology of F₂ plants. The various combinations of the chromosomes of the translocation complex expected in the F₂ plants were derived and their expected frequencies calculated by taking into account chiasma formation at the chromosome ends and various restrictions of gamete viability. In none of the three hybrid families F₂ individuals were found to produce trivalents as the most complex chromosome configuration, indicating that one type of gamete derived from adjacent separation was not formed. The 11 ratio between F₂ plants having only bivalents (2II) and those with F ₁-like quadrivalent configuration R(c), expected when gametes resulting from alternate separation are fully functional, with the exclusion of other types, was not found. That ratio 2II/R(c), was 2-1/3 in the various families. The cytology of selected F₃ individuals basically followed the predictions based on chromosome association in the F₂.     

THE RELATIONSHIPS BETWEEN MALE AND FEMALE FERTILITY AND AMONG TAXA IN DIPLOID WHEATS

Pollen stainability appears to be a reliable indication of the ultimate seed set in diploid interspecific hybrid and backcross populations in Triticum L. The correlation between percent pollen stained and number of seeds set is positive and highly significant (r = 0.92). Estimates of male and female fertility in the hybrids and backcrosses are interpreted to indicate that the domesticated diploid Triticum monococcum L. and wild diploid T. boeoticum Boiss. em. Schiem, are one and the same species, and that T. urartu Tum. is not a variety of monococcum or boeoticum, but rather a separate species. The F₁ hybrids and backcrosses between monococcum and boeoticum are normally male and female fertile. The F₁ hybrids between monococcum and urartu are completely sterile and complete to partial sterility exists in backcrosses.     

Non-reductional meiosis in aTriticum turgidum ×Aegilops longissima hybrid and in backcrosses of its amphidiploid withT. turgidum (Poaceae)

Meiosis was studied in aT. turgidum ×Ae. longissima hybrid (ABS¹, 2n = 21) and in backcrosses of its amphidiploid toT. turgidum. Analysis of PMCs of the hybrid showed that non-reductional meiosis led to the production of a large number of non-reduced male gametes. The hybrid showed high seed set. All progeny had 2n = 42. The BC₁ plants (2n = 35, AABBS¹) showed the expected meiotic pairing of 14^II + 7^I. At anaphase I, univalents behaved in a non-reductional way. The possible role of meiotic non-reduction is discussed in terms of the evolution of theTriticum-Aegilops complex.     

Development of a ploidy series from a single interspecific Trifolium repens L.Ā . nigrescens Viv. F1 hybrid

 The objective of the current research was to generate a ploidy series of backcross progenies from a single triploid (2n=3x=24) Trifolium repens×T. nigrescens F₁ hybrid (3x H-6909-5). The 3x H-6909-5 plant was highly sterile and produced no seeds from approximately 3000 reciprocal backcrosses to both parental species. Chromosome doubling by an in vitro colchicine method resulted in a marked increase in fertility. Pollen stainability was increased from 9.9% in 3x H-6909-5 to an average of 89.2% (range 87.7–90.9%) in the three chromosome-doubled 6x H-6909-5 plants. Subsequent backcrosses of 6x H-6909-5 and interbreeding of backcross derivatives resulted in an array of fertile hybrids at 4x, 5x and 7x levels and some aneuploids. The occurrence of 7x BC₁F₁ progeny from the T. repens×6x H-6909-5 (4x×6x) cross is the first unequivocal evidence of functional female 2n gametes in white clover. Meiotic pairing in F₁ and BC₁F₁ progeny indicated the presence of allosyndetic pairing, suggesting that genetic exchange between the two species is possible. Received: 17 October 1996 / Accepted: 8 November 1996     

Convergent evolution of Darwin's finches caused by introgressive hybridization and selection

Abstract Between 1973 and 2003 mean morphological features of the cactus finch, Geospiza scandens, and the medium ground finch, G. fortis, populations on the Galápagos island of Daphne Major were subject to fluctuating directional selection. An increase in bluntness or robustness in the beak of G. scandens after 1990 can only partly be explained by selection. We use 16 microsatellite loci to test predictions of the previously proposed hypothesis that introgressive hybridization contributed to the trend, resulting in genes flowing predominantly from G. fortis to G. scandens. To identify F₁ hybrids and backcrosses we use pedigrees where known, supplemented by the results of assignment tests based on 14 autosomal loci when parents were not known. We analyze changes in morphology and allelic composition in the two populations over a period of 15–20 years. With samples that included F₁ hybrids and backcrosses, the G. scandens population became more similar to the G. fortis population both genetically and morphologically. Gene flow between species was estimated to be three times greater from G. fortis to G. scandens than in the opposite direction, resulting in a 20% reduction in the genetic difference between the species. Nevertheless, removing identified F₁ hybrids and backcrosses from the total sample and reanalyzing the traits did not eliminate the convergence. The two species also converged in beak shape by 22.2% and in body size by 45.5%. A combination of introgressive hybridization and selection jointly provide the best explanation of convergence in morphology and genetic constitution under the changed ecological conditions following a major El Niño event in 1983. The study illustrates how species without postmating barriers to gene exchange can alternate between convergence and divergence when environmental conditions oscillate.     

Development of Yellow Mosaic Virus (YMV) resistance linked DNA marker in <Emphasis Type="Italic">Vigna mungo</Emphasis> from populations segregating for YMV-reaction

Yellow mosaic virus, YMV, causes one of the most severe of biotic stresses in Vignas, an important group of pulse crops. The viral disease is transmitted through the white fly, Bemicia tabaci, and the yield of the plants is affected drastically. YMV-tolerant lines, generated from a single YMV-tolerant plant identified in the field within a large population of the susceptible cultivar T-9, were crossed with T-9, and F₁, F₂ and F₃ progenies raised. The different generations were phenotyped for YMV-reaction by forced inoculation using viruliferous white flies. A monogenic recessive control of YMV-tolerance was revealed from the F₂ segregation ratio of 3:1 (susceptible: tolerant), which was confirmed by the segregation ratio of the F₃ families. Of 24 pairs of resistance gene analog (RGA) primers screened, only one pair, RGA 1F-CG/RGA 1R, was found to be polymorphic among the parents. Selected F₂ individuals and F₃ families were genotyped with the polymorphic RGA primer pair and the polymorphism was found to be linked with YMV-reaction. This primer pair amplified a 445bp DNA fragment only from homozygous tolerant and the heterozygous lines. The 445bp marker band was sequenced and named 'VMYR1'. The predicted amino acid sequence showed highly significant homology with the NB-ARC domain present in several gene products involved in plant disease resistance, nematode cell death and human apoptotic signaling. To the best of our knowledge, this is the first report of YMV-resistance linked DNA marker development in any crop species using segregating populations. This YMV-resistance linked marker is of potential commercial importance in resistance breeding of plants.     

Distribution and characterization of hexose 6-phosphate dehydrogenase in trout

Electrophoretic analysis of trout liver extracts indicates that an autosomal gene coding for hexose 6-phosphate dehydrogenase (H6P D) in lake trout, Hpd-L, is monomorphic. In brook trout, the gene is polymorphic, having at least three genetic variants termed Hpd-B ¹, Hpd-B², and Hpd-B³. F ₁ hybrid splake exhibit the three expected phenotypes resulting from Hpd-L/Hpd-B ¹, Hpd-L/Hpd-B², and Hpd-L/Hpd-B ³ genotypes. Trout H6P D is ostensibly a dimer of mol wt 230,000. The characteristics of trout H6P D, including substrate specificity, genetic polymorphism, electrophoretic characteristics, subcellular localization, and tissue distribution are in close agreement with results obtained for mammalian H6P D. We suggest that trout and mammalian H6PDs, X-linked mammalian G6P D, and autosomal avian G6P D arose from a common ancestral type G6P D.The research was supported in part by National Science Foundation grant GB-7271 and by United States Public Health Service Predoctoral Fellowship 4 FO1 GM41704-03.     

Evolution of postzygotic reproductive isolation in galliform birds: analysis of first and second hybrid generations and backcrosses

The process of speciation is a crucial aspect of evolutionary biology. In this study, we analysed the patterns of evolution of postzygotic reproductive isolation in Galliformes using information on hybridization and genetic distance among species. Four main patterns arose: (1) hybrid inviability and sterility in F₁ hybrids increase as species diverge; (2) the presence of geographical overlap does not affect the evolution of postzygotic isolation; (3) the galliforms follow Haldane's rule; (4) hybrid inviability is higher in F₂ than in F₁ hybrids, but does not appear to be increased in the backcrosses. This study contributes to the growing evidence suggesting that the patterns of evolution of postzygotic isolation and the process of speciation are shared among avian groups (and animals in general). In particular, our results support the notion of F₂ hybrid inviability as being key for the maintenance of species genetic integrity when prezygotic isolation barriers are overcome in closely related species, in which postzygotic isolation in the F₁ hybrid might still not be fully developed. To the contrary, hybrids from backcrosses did not show serious inviability problems (at least not more than F₁ hybrids), demonstrating that they could generate gene flow among bird species. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110 , 528–542.     

Inheritance of Traits Controlled by Odd Chromosomes Using Data on Transmission of Monosomic Addition S<Superscript>t</Superscript> Chromosome of the <Emphasis Type="Italic">Elymus Sibiricus</Emphasis> Genome

On the basis of the winter bread wheat cultivar Obryi, two independent disomic addition lines BC₁₂F_∞ with the chromosome of the E. sibiricus S^t genome are created. A practical algorithm for determining the probabilities of transmission of the odd chromosome separately through male and female gametes in selfpollination of hemizygous hybrids from the equation p²–(1 + f₁–f₄) × p + f₁ = 0 is proposed, where p is the probability of the formation of viable gametes with the considered chromosome and f₁ and f₄ are the empirical frequencies of the corresponding homozygotes with and without the trait. The probability of transmission of an alien univalent chromosome through pollen (p_♂) is associated with the frequency of its transmission through the egg cell (p_♀) in backcrosses and in self-pollination (1–f₄) by the equation p_♂ = 1–f₄/(1–p_♀). The calculated empirically dependent estimates of the probabilities of transmission of the added chromosome through the egg cell p_♀ = 18.7% and through pollen p_♂ = 4.3% correspond to the empirical frequencies obtained for backcrosses. The coefficients of the gamete selection V_♀ = 0.748 and V_♂ = 0.172 are calculated, and the expected segregation for the alien trait controlled by a dominant gene located in the added chromosome is determined—with the trait: without the trait is 0.222: 0.778 in F₂; 0.187: 0.813 in equational and 0.043: 0.957 in certational backcrosses.     

Hybridization of two forms of ninespine stickleback,Pungitius pungitius and P. platygaster,under experimental conditions and an attempt to predict the consequences of their contact in nature

Synopsis In aquaria and in ponds crosses were made between two closely related allopatric forms of ninespine stickleback of the genus Pungitius (P. pungitius and P. platygaster). There were no ethological isolation mechanisms; F₁, F₂ and F₃ hybrids are fertile; experimental hybrid stocks as well as backcrosses maintain themselves for at least three generations. F₁ hybrids are readily distinguishable from parental forms by a combination of morphological characters; F₂ hybrids grown in the north near the Arctic circle, are morphologically similar to P. pungitius. The southern form P. platygaster does not survive in northern conditions; eggs of the northern form P. pungitius die at a temperature of 28°C, normal for the spawning of the southern form. By 9 out of 19 studied morphological characters there are distinctions between the two forms. Therefore, ecologically and morphologically the two forms have diverged rather significantly. On the other hand, ethologically and genetically (as judged by the absence of postmating isolating mechanisms) the forms have diverged only slightly. As a result, the differences between the two forms are considered not to exceed the subspecies or at least semispecies rank.In recent years, due to a disturbance of the ranges of both these forms in Eurasia, a tendency for a secondary contact between P. pungitius and P. platygaster appeared in the basins of the rivers Volga and Irtysh. In the immediate future the two forms are expected to come into contact. From our data we predict possible introgressive hybridization between the northern and sourthern forms in natural conditions.     

A CYTOGENETIC STUDY OF A THREE-PAIRED RACE OF HAPLOPAPPUS GRACILIS

Haplopappus gracilis (Nutt.) Gray is an aneuploid species of Compositae of wide distribution in the southwestern U.S.A. and northern Mexico. Except for 2 types of supernumeraries, cytological samples from throughout its range have regularly shown 2 pairs of chromosomes. However, recent collections from a small area in south-central Arizona had 2n = 4, 2n = 5, and 2n = 6 in the same population. The 2n = 5 plants were hybrids between the 2n = 4 and 2n = 6 types. Both natural and artificial F₁ hybrids show preferential disjunction from a trivalent and produce genetically balanced gametes with n = 2 and n = 3. In backcrosses of the F₁ to n = 2 plants, a 1:1 ratio of 2n = 4 and 2n = 5 plants was obtained. The possible mechanisms for the evolution of n = 2 from n = 3 and vice versa are discussed.     

Chronic photoinhibition in seedlings of tropical trees

Seedlings of five canopy species of tropical trees from Costa Rica and Puerto Rico were grown in full shade (midday range of photosynthetic photon flux density [PPFD], 100–140 μmol m^?2 s^?1), partial shade (midday PPFD, 400–600 μmol m^?2 s^?1) and full sun (midday PPFD, 1 500–1 800 μmol m^?2 s^?1) for 3 months. The species were Ochroma lagopus (Bombacaceae), a pioneer species; Inga edulis (Fabaceae), found in secondary forest; and Dipteryx panamensis (Fabaceae), Hampea appendiculata (Malvaceae), and Manilkara bidentata (Sapotaceae), three species characteristic of primary forest. After the plants were placed in the dark overnight, chlorophyll fluorescence characteristics were measured for recently expanded and mature leaves. The ratio of variable fluorescence to maximum fluorescence (F_v/F_m) was used to estimate the degree of chronic photoinhibition. Only individuals of one species, Dipteryx panamensis, showed significant depression of F_v/F_m after long-term exposure to full sun. The depression was highly correlated with quantum yield of O₂ evolution which also declined after exposure to full sun. The decline may have been related to foliar N concentration. Although all plants were supplied with ample nutrients, foliar N did not increase significantly for Dipteryx seedlings in full sun, whereas it did for Ochroma and Inga. Leaf age affected F_v/F_m only in the cases of Manilkara, where it was slightly lower in recently expanded leaves, and of Dipteryx where it interacted with the effects of light regime. We conclude that chronic photoinhibition is not common in seedlings of canopy trees of tropical rain forests except when availability of mineral nutrients may be limiting.     

ORIGIN OF FRAGARIA POLYPLOIDS. II. UNREDUCED AND DOUBLED-UNREDUCED GAMETES

Offspring from natural hybrids between octoploid Fragaria chiloensis (2n = 56) and diploid F. vesca (2n = 14) backcrossed under natural conditions to F. chiloensis were studied. The natural F₁ hybrids themselves were of three kinds: (1) The expected pentaploids which resulted from the union of normally reduced gametes of diploid F. vesca and octoploid F. chiloensis; (2) A hexaploid F₁ hybrid which resulted from the union of an unreduced gamete from diploid F. vesca with a normally reduced gamete from octoploid F. chiloensis; and (3) A 9-ploid F₁ hybrid which probably arose from the union of an unreduced gamete of the octoploid F. chiloensis with a normally reduced gamete of diploid F. vesca. The progenies that resulted from the natural backcrossing of each of the three sorts of F₁ hybrids to F. chiloensis were as follows: The pentaploid F₁ hybrids (2n = 35) yielded mostly 9-ploid offspring from unreduced 5X gametes; a relatively high percentage of 14-ploid plants arising from doubled-unreduced 10 X gametes and a few 2N = ±46 aneuploids from reduced gametes. The hexaploid F₁ hybrid (2n = 42) on backcrossing yielded over 50% 10-ploid offspring with the rest 2n = ±50 aneuploids from reduced gametes. The 9-ploid F₁ hybrid (2n = 63) on backcrossing yielded mostly aneuploids normally distributed about a modal 2n = 59 chromosome class resulting from a 31 chromosome gamete, with a few 2n = 56 and 2n = 63 euploids. The 9-ploids may facilitate diploid Å octoploid introgression. Screening of the open-pollinated offspring from F. chiloensis revealed almost 2% 12-ploid (2n = 84) offspring from the union of the reduced and unreduced F. chiloensis gametes. The probable genomic constitution of the observed novel ploidy levels and those that theoretically may be generated from the known hybrids are presented. The origin of the existing polyploids from diploids through simple unreduction is postulated.     

Analysis of transgene(s) (psy+crtI) inheritance and its stability over generations in the genetic background of indica rice cultivar Swarna

Rice the major staple food crop which feeds more than half of the world’s population but, lacks pathway to synthesize and accumulate provitamin A in endosperm therefore rice eaters particularly children, and pregnant women suffer due to vitamin A deficiency. The pathway for provitamin A synthesis in rice endosperm has been engineered and transgenic rice lines have yellow endosperm, called ‘Golden Rice’. The present study aimed at studying the inheritance of transgene(s) in six transgenic events of ‘Golden Rice’ and transfer of provitamin A trait from transgenic lines to a widely grown mega rice variety Swarna. The events E1, R1 and W1 showed normal Mendelian inheritance in F₂, BC₁F₁ and BC₁F₂ generations. The event W1 was studied in BC₁F₃ as well and showed normal Mendelian inheritance of 3:1. The inheritance pattern in L1 event in BC₁F₁ and BC₁F₂ showed normal Mendelian inheritance following expected ratio 1:1 and 3:1 respectively. The two events G1 and T1 showed distorted segregation in BC₁F₂ and BC₂F₂ respectively in Swarna genetic background. In G1 event, transgene inheritance showed segregation distortion in BC₁F₂ in favour of transgene negative plants. In T1 event, inheritance followed expected Mendelian segregation in BC₁F₁, BC₂F₁ and BC₂F₂, generations. However, when tested against co-dominant inheritance 1:2:1 pattern in BC₂F₂, segregation distortion was observed with less than the expected transgene homozygotes. While against 3:1 ratio, it showed the expected segregation pattern in BC₂F₂ generation. Segregation distortion probably due to differential transmission of transgene positive/negative gametes through either/both parents which needs further study.     

Chlorophyll fluorescence performance of sweet almond [Prunus dulcis (Miller) D. Webb] in response to salinity stress induced by NaCl

One-year old sweet almond (Prunus dulcis) seedlings were submitted to four levels of salt stress induced by NaCl, namely 0.3, 0.5, 0.7, and 1.0 S m⁻¹. Effects of salt stress on a range of chlorophyll (Chl) fluorescence parameters (Chl FPs) and Chl contents were investigated in order to establish an eco-physiological characterization of P. dulcis to salinity. Salt stress promoted an increase in F₀, F_s, and F₀/F_m and a decrease in F_m, F′_m, F_v/F_m, q_P, ΔF/F′_m, F_v/F₀, and UQF_(rel), in almost all Chl fluorescence yields (FY) and FPs due to its adverse effect on activity of photosystem 2. No significant changes were observed for quenchings q_N, NPQ, and q_N(rel). The contents of Chl a and b and their ratio were also significantly reduced at increased salt stress. In general, adverse salinity effects became significant when the electric conductivity of the nutrient solution (EC_n) exceeded 0.3 S m⁻¹. The most sensitive salt stress indicators were F_v/F₀ and Chl a content, and they are thus best used for early salt detection in P. dulcis. Monitoring of a simple Chl FY, such as F₀, also gave a good indication of induced salt stress due to the significant correlations observed between the different Chl FYs and FPs. Even essential Chl FYs, like F₀, F_m, F′_m, and F_s, and mutually independent Chl FPs, like F_v/F₀ and q_P, were strongly correlated with each other.     

Community structure of canopy arthropods associated to Quercus crassifolia×Quercus crassipes complex

Quercus crassifolia and Q. crassipes are dominant species in temperate forests of central Mexico and hybridize between each other when they occur in sympatry. Oak canopies contain a considerable portion of arthropod diversity and the hybrid zones can provide new habitats to epiphyte fauna. We tested if the establishment of hybrids in contact zones with their parental hosts increases the species diversity of canopy arthropods assuming that hybrid trees constitute new genotypes of potential new habitats to small organisms. We examined the effect of hybridization on some community structure parameters (diversity, composition, similarity and density of arthropod fauna) of canopy arthropods compared to their parental species in a hybrid zone located in central Mexico. We employed 17 leaf morphological traits and six diagnostic RAPD primers to identify parental and hybrid plants. The RAPDs marker showed unidirectional introgression towards Q. crassifolia, and were detected hybrid (F₁), backcrosses and introgression individual trees. In total, 30 oak canopies were fogged during rainy and dry season. We recognized 532 taxa of arthropods belonging to 22 orders associated with tree canopies. The taxonomic status of host‐trees may be an important factor in the arthropod community structure and that seasonality (dry and rainy) is not a factor that could modify their organization. Trees of Q. crassipes registered the highest densities of arthropod fauna followed by hybrid hosts (F₁); trees originated by backcrosses towards Q. crassifolia registered a significant less arthropod density than F₁ hybrids; and trees of Q. crassifolia had the lowest density. Hybrid plants and Q. crassipes individuals had higher diversity (H′) of arthropods than Q. crassifolia plants. Hybrid plants had also more rare species in both seasons in comparison with parental species. This study suggests that hybrid oaks act as a center of biodiversity by accumulating arthropods of both parental and different species including a considerable number of rare species.     

Photon yield of O2 evolution and chlorophyll fluorescence characteristics at 77 K among vascular plants of diverse origins

Photon yields of oxygen evolution at saturating CO₂ were determined for 44 species of vascular plants, representing widely diverse taxa, habitats, life forms and growth conditions. The photonyield values on the basis of absorbed light ( ^a) were remarkably constant among plants possessing the same pathway of photosynthetic CO₂ fixation, provided the plants had not been subjected to environmental stress. The mean ^a value ±SE for 37 C₃ species was 0.106±0.001 O₂·photon^-1. The five C₄ species exhibited lower photon yields and greater variation than the C₃ species ( ^a=0.0692±0.004). The ^a values for the two Crassulaceanacid-metabolism species were similar to those of C₃ species. Leaf chlorophyll content had little influence on ^a over the range found in normal, healthy leaves. Chlorophyll fluorescence characteristics at 77 K were determined for the same leaves as used for the photon-yield measurements. Considerable variation in fluorescence emission both at 692 nm and at 734 nm, was found 1) among the different species; 2) between the upper and lower surfaces of the same leaves; and 3) between sun and shade leaves of the same species. By contrast, the ratio of variable to maximum fluorescence emission at 692 nm (F_v/F_M, 692) remained remarkably constant (The mean value for the C₃ species was 0.832±0.004). High-light treatments of shade leaves resulted in a reduction in both ^a and the F_v/F_M, 692 ratio. The extent of the reductions increased with time of exposure to bright light. A linear relationship was obtained when ^a was plotted against F_v/F_M, 692. The results show that determinations of the photon yield of O₂ evolution and the F_v/F_M, 692 ratio can serve as excellent quantitative measures of photoinhibition of overall photosynthetic energy-conversion system and of photochemistry of photosystem II, respectively. This is especially valuable in field work where it is often impossible to obtain appropriate controls.Abbreviations and symbols CAM Crassulacean acid metabolism - PFD photon flux density (photon fluence rate) - PSI, PSII photosystem I, II - F_o, F_M, F_v instantaneous, maximum, variable fluorescence emission - absorptance - ^a photon yield (absorbed light) - ⁱ photon yield (incident light) C.I.W.-D.P.B. Publication No. 923     

LOSS OF HYBRID LETHALITY DURING BACKCROSS PROGRAMS INVOLVING CHLAMYDOMONAS EUGAMETOS AND CHLAMYDOMONAS MOEWUSII (CHLOROPHYCEAE)1

Wild-type strains of the interfertile species Chlamydomonas eugametos (UTEX 9 and 10) and Chlamydomonas moewusii (UTEX 96 and 97) male readily and reciprocally; however, considerable lethality occurs among F₁ hybrid meiotic products. We prepared two hybrid backcross lineages using C. eugametos and C. moewusii. One lineage began with the cross C. eugametos mating-type-plus (mt⁺) × C. moewusii mating-type-minus (mt^?). An F₁ mt⁺ hybrid from this cross was back-crossed to C. moewusii mt^?, and a B₁ mt⁺ hybrid was recovered. The B₁ hybrid was again backcrossed to C. moewusii mt^?, and this process was repeated through the fifth backcross. The other backcross lineage began with the reciprocal cross C. moewusii mt⁺× C. eugametos mt^? and employed C. eugametos as the recurring mt^? parent. This lineage also was continued through the fifth backcross. Meiotic product survival in the reciprocal interspecific crosses was less than 10%. In successive back-cross generations associated with both lineages, this value increased progressively to a maximum of 85–90%, the level observed for the intraspecific crosses. These results are consistent with the hypothesis that multiple genetic differences exist between C. eugametos and C. moewusii and that these are the major source of meiotic product lethality associated with the interspecific crosses. The inheritance of chloroplast genetic markers for resistance to streptomycin (sr-2) and for resistance to erythromycin (er-nM1) was also scored w the interspecific crosses and in the backcrosses. Most hybrid zygospores transmitted the resistance markers of the mt⁺ parent only, or of both parents, with the former zygospore type being more common. Although the intraspecific C. eugametos and C. moewusii crosses differ conspicuously with respect to the fraction of zygospores which transmit chloroplast genetic markers of both parents, the inheritance of chloroplast genetic markers in the interspecific crosses and backcrosses at' scribed here failed to clarify the genetic basis for this difference.     

Comparative genetic linkage maps of <Emphasis Type="Italic">Eucalyptus grandis</Emphasis>, <Emphasis Type="Italic">Eucalyptus globulus</Emphasis> and their F<Subscript>1</Subscript> hybrid based on a double pseudo-backcross mapping approach

Comparative genetic mapping in interspecific pedigrees presents a powerful approach to study genetic differentiation, genome evolution and reproductive isolation in diverging species. We used this approach for genetic analysis of an F₁ hybrid of two Eucalyptus tree species, Eucalyptus grandis (W. Hill ex Maiden.) and Eucalyptus globulus (Labill.). This wide interspecific cross is characterized by hybrid inviability and hybrid abnormality. Approximately 20% of loci in the genome of the F₁ hybrid are expected to be hemizygous due to a difference in genome size between E. grandis (640 Mbp) and E. globulus (530 Mbp). We investigated the extent of colinearity between the two genomes and the distribution of hemizygous loci in the F₁ hybrid using high-throughput, semi-automated AFLP marker analysis. Two pseudo-backcross families (backcrosses of an F₁ individual to non-parental individuals of the parental species) were each genotyped with more than 800 AFLP markers. This allowed construction of de novo comparative genetic linkage maps of the F₁ hybrid and the two backcross parents. All shared AFLP marker loci in the three single-tree parental maps were found to be colinear and little evidence was found for gross chromosomal rearrangements. Our results suggest that hemizygous AFLP loci are dispersed throughout the E. grandis chromosomes of the F₁ hybrid.Communicated by O. Savolainen