CLINES FOR HYBRID DYSFUNCTION IN A GRASSHOPPER HYBRID ZONE

Two subspecies of the grasshopper Chorthippus parallelus meet in the Pyrenees forming a hybrid zone several kilometers wide. Crosses between the two pure taxa result in sterile male offspring and normal females (i.e., Haldane's rule applies). However, no such dysfunction has been detected in hybrid males collected through the center of the hybrid zone. By assessing the level of dysfunction in the offspring of reciprocal crosses, it was possible to map clines for the genes responsible for dysfunction through the zone. This analysis shows that there is no abrupt transition between incompatible genomes in the field. Crosses were also made between females collected from a transect spanning the hybrid zone and pure males of both subspecies. This reveals noncoincident clines for dysfunction near the center of the hybrid zone such that the dysfunction expressed in the offspring of these crosses is less than expected from simple models. More complex models involving interaction among genes must be invoked. Also, the possibility exists that since the postglacial contact of these two grasshopper taxa, hybrid dysfunction has become ameliorated by the evolution of modifiers. This hybrid zone is thought to be a tension zone, maintained by a balance between selection against hybrid genotypes and dispersal into the zone center. The lessening of hybrid disadvantage over time through the breakdown of epistatic interactions by recombination or through modification could account for the general lack of dysfunction in field collected hybrids today.     

MORPHOLOGY OF SACCHARUM-SORGHUM HYBRID DERIVATIVES

The intergeneric hybrid between sugarcane (Saccharum sp.) and sorghum (Sorghum bicolor (L.) Moench.) was produced by pollinating sugarcane inflorescences of clone F 36-819 (2n = ca. 112) with diploid (2n = 20) ‘Rex’ sorghum pollen. The backcross four (BC₄) population, produced by back-crossing twice with diploid and then twice more with tetraploid (2n = 40) ‘Rio’ sorghum, combined 40 Sorghum and 4-10 Saccharum chromosomes. Different BC₄ individuals were morphologically distinct within and between chromosome complexes, indicating that their genomes must comprise different combinations of Sorghum and Saccharum chromosomes. Selfing for one or more generations results in almost complete elimination of Saccharum chromosomes and morphologically modified sorghums are recovered. Modified sorghums have various combinations of Saccharum characteristics superimposed on a basic Sorghum morphology. The presence of Saccharum characters in some of the recovered sorghums with 2n = 40 chromosomes indicates the potential for a gene transfer from sugarcane to sorghum.     

MITOCHONDRIAL INHERITANCE PATTERNS ACROSS A COTTONWOOD HYBRID ZONE: CYTONUCLEAR DISEQUILIBRIA AND HYBRID ZONE DYNAMICS

In this study we examine the cytoplasmic inheritance patterns of an interspecific hybridizing population of Fremont and narrowleaf cottonwoods, using mitochondrial DNA. Three mitochondrial probes showing polymorphisms were used to distinguish between trees of known nuclear inheritance. Every tree screened had only one cytoplasmic genotype, either Fremont or narrowleaf. Thus, these results demonstrate that mitochondria are uniparentally inherited in these trees. Previous studies of the nuclear inheritance of this interspecific hybridizing population of cottonwood trees indicated an asymmetry in the frequency of parental genes. Using mitochondrial markers we tested one hypothesis potentially responsible for this asymmetric distribution (i.e., trees of mixed genotypes will be sterile or will not survive if their cytoplasm is derived from one or the other parent). Our results, however, show that both Fremont and narrowleaf mitochondrial markers are found in trees with mixed nuclear genotypes. Thus, nuclear-cytoplasmic incompatibilities do not appear to account for the asymmetric distribution of nuclear genotypes within the hybrid swarm. An alternative explanation for the observed asymmetric distribution of nuclear genotypes is advanced. Although nuclear-cytoplasmic incompatibilities do not appear to explain the asymmetric distribution of nuclear alleles within the hybrid zone, nonrandom associations between nuclear and cytoplasmic genotypes do exist. For example, all F₁ hybrids had Fremont mitochondrial genotypes. Furthermore, backcrosses between F₁ hybrid and narrowleaf trees have a higher than expected proportion of heterozygous loci and a higher than expected proportion of Fremont mitochondria. We propose that seeds, seedlings, or trees with high proportions of heterozygous loci are at a disadvantage unless they also have the Fremont mitochondrial genotype. While it is generally difficult to infer dynamic processes from static patterns, studies such as ours enable one to gain new insights to the dynamics of plant hybrid zones. A hybridization pattern of decreasingly complex backcrosses as one proceeds from higher to lower elevation within the hybrid swarm, a residue of Fremont cytoplasmic DNA within the pure narrowleaf population, and the unidirectional nature of these crosses suggest that the narrowleaf population may be spreading down the canyon and the Fremont population receding. The eventual fate of the hybrid zone, in relation to these processes, is discussed.     

柑橘体细胞杂种有性后代的创造及杂种鉴定

以异源四倍体柑橘体细胞杂种[哈姆林甜橙(Citrus sinensis Osbeck) 粗柠檬(C.jambhiri Luss)]为父本与二倍体单胚类型宜本杂4号[华农本地早橘(C.reticulata Blanco)×宜昌橙(C.ichangen- sis Swingle)]杂交,采用胚抢救技术获得了110株有性后代植株,通过染色体计数及倍性分析仪鉴定,其中93株植株为3倍体,余下的17林植株为2倍体。RAPD分析表明:获得的有性后代植株均为杂种。     

几组杂交组合的杂交稻及其亲本光合特性的比较研究

研究了３组杂交组合的杂交稻及其亲本的光合特性。结果表明,杂交稻叶片单位鲜重的光合色素含量明显高于它们的亲本。它们的ＰＳⅡ活性、ＰＳⅡ原始光能转化效率和叶片的潜在光合作用量子转化效率,以及光合电子传递率和对两个光系统之间激发分配的调节能均高于各自的亲本。此外,上述各光合功能又以母本优于父本。根据所得的实验结果,认为在配置杂交组合时,选择具有优良光合功能的母本,是获得在生产上可推广应用的杂交稻的重要途径之一。     

HYBRID ZONES WITH DOBZHANSKY-TYPE EPISTATIC SELECTION

Dobzhansky's model of epistatic selection assumes that viable genotypes form “clusters” in genotype space so that populations can evolve from one state to a reproductively isolated state following a “ridge” of well-fit genotypes without crossing any deep adaptive valleys. Recently, the importance of Dobzhansky-type models in evolutionary studies has been reemphasized by Gavrilets (1997a) and Gavrilets and Gravner (1997) who argue that the existence of “ridges” of well-fit genotypes connecting reproductively isolated genotypes is actually a general property of multidimensional adaptive landscapes. Using rigorous techniques and numerical simulations, I analyze clines in the frequencies of selected and neutral alleles maintained by a balance of migration and Dobzhansky-type epistatic selection acting on two diallelic loci. I show that Dobzhansky-type epistatic selection can build up a very strong barrier to neutral gene flow. I describe properties of clines that are indicative of Dobzhansky-type selection.     

A PLUCHEA HYBRID FROM THE PACIFIC

Two species of Pluchea have been introduced into the Hawaiian Islands in the present century. These are P. indica, native to southeastern Asia, and P. odorata, native to the western hemisphere. Both are erect shrubs. Within the past 30 years a third Pluchea taxon, a sprawling shrub morphologically distinct from the two species, has appeared in the Hawaiian Islands and on other islands in the tropical Pacific. On the basis of morphology, fertility, meiotic behavior, and other evidence, it is concluded that the third taxon is a hybrid between the two species. The hybrid is highly sterile; it has a limited capacity for asexual reproduction. Metaphase I configurations for both species were found to be 10_II; the most frequent configuration in the hybrid was 8_II + 4_I. The hybrid is given a binomial.     

红金麻——木槿属一个新杂种

<正> 红金麻 新杂种 Hibiscus×hongjinma Y. T. Chang, hybr. nov. H. cannabinus L. ♀× H. radiatus Cav.♂ Hybrida hortensis ( 2n=30+6sat) θ Hibisco cannabino L. ( 2n=32+4sat) et H. ranndiato Cav. (2n=70+2sat) exorta, Habitu illo simil, characteribus ad hoc magis accedit; ab illo differt floribus purpureis, folio prope basin 3—5     

THE RIBOSOMAL RNA OF HAMSTER-MOUSE HYBRID CELLS

The ribosomal RNA (rRNA) of a series of hamster-mouse somatic cell hybrids was studied. Mouse 28S rRNA was separated from its hamster counterpart by a two-step procedure involving sucrose gradient centrifugation of ribosomes and polyacrylamide gel electrophoresis of rRNA. Both hamster and mouse types of rRNA were synthesized in the 11 hybrids tested, including hybrids containing only about one-half the haploid number of either mouse or hamster chromosomes. It appears that, for both hamster and mouse rRNA, when the chromosomes of one species constituted the majority of the chromosomes of a hybrid, a disproportionately higher percentage of rRNA of that species was present in the hybrid. Some hybrid clones, having a majority of mouse chromosomes, had a mouse rRNA cell concentration approximately four to five times higher than the concentration expected from linear extrapolation of the value found for the mouse parental cell line.     

几组有优和无优杂交组合中杂交稻及其亲本光合功能的比较

研究了３个杂交组合的杂交稻及其亲本的光合功能。实验结果表明,杂交稻的叶绿素（Ｃｈｌ）和类胡萝卜素（Ｃａｒ）的含量高于其亲本。在２组有优杂交组合中,杂交稻的ＰＳⅡ活性、ＰＳⅡ原初光能转化效率和叶片潜在光合作用量子转化效率,以及光合电子传递能力和对两个光系统之间激发能分配的调节能力,均高于其各自的亲本。此外,上述各光合功能又以母本优于父本。然而,在无优组合中,上述的光合功能却是父本高于母本和它们的子一代。根据所得的实验结果,进一步证实在配置杂交组合时,选择具有优良光合功能的母本,是获得在生产上可推广应用的杂交稻的重要途径之一。     

杂交鲤的生产和推广

1974年湖北省水生生物研究所送给我场一批散鳞镜鲤、龙州镜鲤和兴国红鲤。1975和1976年并派出技术人员到我场协助进行鲤鱼品种间的杂交试验,两年共繁殖了杂交鲤苗713,000多尾,经过饲养,效果良好,深受群众欢迎,并把它誉为“丰鲤”。