Fi Hybrids Research Articles

Chromosomal speciation in a Rassenkreis of Venezuelan spiny rats (genus Proechimys, Rodentia, Echimyidae)

A series of populations of spiny rats (Proechimys) of NW and N Central Venezuela were found to belong to a circle of species and subspecies which surrounds the Andes of M6rida-CordiUera de la Costa mountain axis. This 'Rassenkreis' consists of 6 successive karyomorphs which exhibit a stepped-dine distribution (2n = 42, 44, 46, 48, 50 and 62). Each of the karyomorphs occupies a discrete subarea in an almost continuous territory. Polymorphism is very rare and no intermediate karyotypes have been found in the borders of each subarea. Although contact zones have not been studied in detail, the distribution seems to be mostly parapatric. Areas are 12-100 km apart, in most cases without effective barrier between them. Only 1 Robertsonlan rearrangement accounts for each of the steps from 2n = 44 to 2n = 50. The transition from 2n = 42 to 2n = 44 involves 1 Robertsonian change and 3 pericentric inversions. The transition from 2n = 50 to 2n = 62 requires 6 Robertsonian rearrangements and 2 pericentric inversions. It is inferred that successive populations of the 2n = 44 2n = 50 series may be at an incomplete stage of reproductive isolation by partial reproductive failure of the FI hybrids. Reproductive isolation may be almost complete between the 2n = 42 and 2n = 44 and the 2n = 50 and 2n = 62 karyomorphs. This 'Rassenkreis' is classified as a superspecies, Proechimys [guairae], comprising three allospecies: P. poliopus, P. guairae, and a new unnamed species (Barina's~ sp. n.). Within P. guairae 4 subspecies (or semispecies) are recognized, 2 of which are named (P. guairae guairae and P. guairae ochraceus). It is postulated that differentiation within this superspecies occurred during the Late Pleistocene. The tentative interpretation is advanced that rapid colonization occurred over most of the present territory by a chromosomically uniform invading stock, which subsequently became differentiated by parapatric chromosome raciation. This was followed by further chromosome differentiation in peripheral isolates which formed in unfavorable, dry climatic phases. During humid climatic phases the ranges expanded and zones of secondary contact with the original populations were established.

The relationship between heterozygosity and heterosis in reproductive traits in mice

ABSTRACTContinuous reciprocal backcrossing results in individuals which are 67% heterozygous, and which, assuming a linear relationship between heterozygosity and heterosis, would be expected to exhibit 67% of the heterosis found in Fi hybrids. An experiment was undertaken to investigate this relationship in the reproductive traits of mice. By crossing and repeated backcrossing in both directions using two unrelated strains of outbred mice, females were produced which were 25, 50, 75 and 100% heterozygous. The crossbred females and the two purebred strains (922 females in all) were mated to a sire line of a genetically distinct strain to standardize foetal heterosis. The reproductive performance of females with different percentage heterozygosity gave estimates of maternal heterosis. A significant positive linear relationship was found between heterozy-gosity and heterosis in litter size, total weight of litter and individual weight of progeny both at birth and at weaning. The relevance of these results for livestock breeding is discussed.

Testing Fi hybrids of Brassica oleracea for sibs by seed isoenzyme analysis

SUMMARYExtracts of seeds of Brassica oleracea L. cultivars were separated by electrophoresis on polyacrylamide gels and stained for 14 enzymes to investigate the possibility of utilising isoenzyme variation to determine sib frequencies in hybrid cultivars. Acid phosphatase (EC 3.1.3.2), cr‐amylase (3.2.1.1), carboxylesterase (3.1.1.1), β‐galactosidase (3.2.1.23), β‐glucosidase (3.2.1.21), leucine aminopeptidase (3.4.11.1) and peroxidase (1.11.1.7) reactions gave discrete bands. Three broad zones of acid phosphatase activity occurred and control of banding patterns in the intermediate zone by a series of co‐dominant alleles was confirmed. The locus is designated acp‐1. Sib frequencies could be estimated in eight of 21 F1, cultivars examined, since their parent lines were homozygous for different acp‐1 alleles. This method of estimation has several advantages and offers a practical alternative to current methods. Routine determination of sib frequencies by using any of the other enzymes investigated is impractical.

Genetic Study of Aggressiveness of two Subspecies ofApis Melliferain Brazil. 2. Time at Which the First Sting Reached a Leather Ball

SummaryThe test used in this study was applied to colonies of Africanized bees (“Apis mellifera adansonii”), of imported Italian bees (Apis mellifera ligustica), of their Fi hybrids, and of the progeny of Fi drones backcrossed to Africanized and Italian queens. The data from Fi colonies indicated dominance of the Africanized bees; other data emphasize the complexity of the method of inheritance of “aggressiveness”.

Genetic Factors Influencing Endogenous Type-C RNA Viruses of Mouse Cells: Control of Viral Polypeptide Expression in the C57BL/10 Strain

Type-C viral polypeptides are expressed at very low levels in C57BL/10 liver and embryo cells as compared to cells of other mouse strains. The 12,000-mol. wt. polypeptide in C57BL/10 cells was purified and shown to possess type-specific antigenic determinants indistinguishable from those of a previously described class of endogenous virus. The very low levels of endogenous virus-specific RNA in cells of C57BL/10 as compared to other strains suggests control of this endogenous virus at the level of transcription. Analysis of viral antigen expression F1 hybrids of C57BL/10 with strains characterized by intermediate and high levels of viral antigen expression indicates that the restriction of this endogenous virus by C57BL/10 cells is not due to simple dominance by a repressor of virus expression.

The Inheritance of Fusarium Wilt Resistance in Watermelon, Citrullus lanatus (Thunb.) Mansf.1

Abstract The inheritance of Fusarium wilt resistance was studied using 3 varieties of watermelon. ‘Summit’ and ‘Charleston Gray’ differed in their level of resistance. ‘Summit, carrying a high level of resistance, acted in a completely dominant manner to the susceptible variety ‘New Hampshire Midget’. On the other hand, ‘New Hampshire Midget’ was partially dominant to the moderately resistant variety ‘Charleston Gray’. The relationship of ‘Summit’ to ‘Charleston Gray’ was not clear, although the data suggested dominance of ‘Summit’ to ‘Charleston Gray’. Three gene models were postulated, a one-locus multiple-allelic model and 2 two-locus models. The deviations from regression (model adequacy) for the multiple-allelic model and for one of the 2-locus models were nonsignificant as shown by the Chi-square test. The coefficient of determination (R2) for the multiple-allelic model was .973 with 5 genetic parameters and for the 2-locus model, R2 = .970 with 4 genetic parameters. Neither model was considered superior to the other. It was suggested that a variety such as ‘Summit’, with a high level of resistance, inherited in a completely dominant manner, could be of value in obtaining Fi hybrids highly resistant to Fusarium wilt, as well as aid in the development of highly resistant open-pollinated varieties.

Nitrate Reductase Activity of Maize Hybrids and Their Parental Inbred1

Field experiments were conducted over a 4‐year period to compare the level of nitrate reductase (NR) activity of corn hybrids with their respective inbred parents. From previous experiments, two groups of inbreds, ranked “high” and “low” with respect to seasonal mean NR activity were selected and used to obtain FI hybrids representing the following categories: “high ✕ high”, “high ✕ low”, and “low ✕ low”.The level of NR activity in the lq hybrid appeared to differ from category to category. In the “high ✕ high” category, the level of NR activity of the hybrids of all three sets of material was lower than the mean of the respective parental inbreds. In the “high ✕ low” category, the hybrids were essentially intermediate in NR activity with respect to their parental inbreds and lower, in all cases, than the higher inbred parent. Two hybrids of the “low ✕ low” category exhibited heterosis for this character. That is, the level of NR activity of the hybrids was significantly higher than the level of the higher inbred parent. Other hybrids of this category were not significantly different in NR activity than the mean of their respective inbred parents or the higher inbred.Since it is accepted that the mechanism of gene action is at the biochemical level through the control of protein synthesis, this work provides some insight into the mode of gene action and outlines an approach to the knowledgeable synthesis of hybrids with specified metabolic characteristics.

The influence of strain, sex and age of mice on infection with Plasmodium berghei

While studying the effect of a Plasmodium berghei infection on the course of leukemia1 in mice it was noted that DBA mice and Fi hybrid mice (C57 Black 9 XDBA d) differed in their resistance to malaria. Of the many investigators who have studied P. berghei in mice, none of whom we are aware has used inbred strains. By the use of such homogeneous strains of mice, differences in the reaction to the infection might become apparent and might lead to profitable investigations concerning genetic and nongenetic influences in infection. The present report deals with the course of P. berghei infections in inbred DBA/2 and C57 Black mice and their Fi hybrids, BD Fi mice.