Haemoglobin polymorphism in atlantic cod(Gadus morhua): Allele frequency variation between yearclasses in a Norwegian fjord stock

A total of 262 specimens (0-, and 1-group) of Atlantic cod(Gadus morhua) representing 4 different yearclasses were caught in Trondheimsfjorden, Norway, during 1977–1984. They were genotyped by agar gel electrophoresis for the polymorphic haemoglobin locusHbI (Sick, 1961). The analyses revealed a highly significant (P=0.0003) heterogeneity ofHbI allele frequencies between yearclasses. The difference in the frequency of theHbI-1 allele between the first (1977) and the last (1983) yearclass amounted to 0.18 (±0.07). The results appear to support recent reports on considerable selection effects atHbI, and stress the unreliability of allele frequencies at this locus for use in studies of the genetic population structure of cod.Contribution No. 229, Biological Station, N-7000 Trondheim, Norway     

Blood catalase and haematocrit values in a breeding colony of Dutch-belted rabbits

Rabbit seminal plasma catalase is much higher than in the semen of other mammals, and differences appear to be inherited. Because of the scarcity of information on rabbit blood catalase and haematocrit in Dutch-belted rabbits, an investigation of possible effects of gender, age and genetics on these variables was undertaken. There were 191 rabbits sampled at 2-3 months, 130 at 12 months and 61 at 18-24 months of age. There was no age effect on the haematocrit values and on blood catalase activity. At 12 months of age males had an average haematocrit value of 44% compared with 40% for females (P < 0.05). Corresponding average catalase values were 431 and 356 units/ml of blood (P < 0.05). Also catalase was measured in the semen and blood of 34 males, and males differed in both their blood and semen catalase activity (P < 0.05). The correlation between the two traits was r = 0.44. Heritability (h2) estimates, based on 231 rabbits were 0.40 for blood catalase activity, and 0.26 for haematocrit. The genetic correlation between the two variables was 0.83 (P < 0.05). These studies are consistent with the literature in that female rabbits have a slightly lower haematocrit value than males, and this is associated with a lower catalase activity. This appears to be the first report of a study that compares rabbit blood catalase in males and females of different ages. Preliminary evidence that differences may have a heritable basis is consistent with previous studies on rabbit semen catalase.     

Genetic and developmental variation of hemoglobin in the deermouse,Peromyscus maniculatus

A genetic investigation of electrophoretic hemoglobin variants of the deermouse, Peromyscus maniculatus, shows three alleles, HbI ^f, HbI^r, and HbI ^o, at a duplicated site controlling the six adult phenotypes. The HbI ^fallele has not been described previously. The hemoglobin locus is not closely linked to the albino locus. Fetal hemoglobin is distinct from any of the adult components and has a slower electrophoretic mobility. The fetal phenotype changes to the adult type between the days 15 and 18 of prenatal life.     

Systematics of Mielichhoferia (Bryaceae: Musci). III. Hybridization between M. elongata and M. mielichhoferiana

Allozyme variation in mixed populations of Mielichhoferia elongata and M. mielichhoferiana was investigated to determine if interspecific hybridization occurs when these two closely related species grow together. Previous research has shown that M. elongata and M. mielichhoferiana can be distinguished by three diagnostic isozyme loci (Gpi-1, Mdh-2, and Mdh-3) at which the two species do not share alleles in 32 allopatric populations from North America and Europe. The present study shows that in five populations from Colorado, Norway, and Sweden, gametophytes resulting from interspecific hybridization can be recognized by recombinant genotypes combining alleles of the otherwise diagnostic loci. A total of 32 multilocus genotypes was found among the 111 individuals sampled, of which 13 were recombinants. The frequency of recombinants ranged from 12% to 35% within populations, and all but one population contained both parental species. Moreover, recombinant genotypes could be accounted for by the allelic constitution of sympatric parents. In two of the populations, more than one hybridization event was necessary to account for the diversity of recombinant genotypes. Twenty-nine of the 32 genotypes detected in this study were restricted to one population each, two occurred in two Swedish populations separated by approximately 14 km, and one occurred in both Sweden and Norway.     

Length‐weight relationships and condition factors of two threatened fishes,Rasbora tawarensis and Poropuntius tawarensis,endemic to Lake Laut Tawar,Aceh Province,Indonesia

Studied were the length‐weight relationships (LWRs) and condition factors of two threatened species, Rasbora tawarensis and Poropuntius tawarensis, endemic to Lake Laut Tawar. The objective of the present study was to establish baseline data on LWRs of the two endemic fishes. The growth pattern for both species was predicted using a Linear Allometric Model (LAM) of length‐weight relationship and Fulton’s condition factor (K_TL) as well as Relative Weight (W_r). The b value of R. tawarensis was relatively similar between females and males, while the b value of female P. tawarensis was slightly higher than in males. Nevertheless, both R. tawarensis and P. tawarensis showed an allometric negative growth pattern. The average K_TL value of R. tawarensis was 1.85 and 1.73 for females and males, respectively, while P. tawarensis had values of 2.44 and 2.41 for females and males, respectively. In addition, the average W_r values of R. tawarensis were 98.9 and 100.45 for females and males respectively, while 102.30 for female and 102.96 for male P. tawarensis, indicating that the condition factors of P. tawarensis were higher than in R. tawarensis.     

Weak geographical structure in sperm morphology across the range of two willow warbler Phylloscopus trochilus subspecies in Scandinavia

Sperm morphology is highly diversified among species and at higher taxonomic levels. In birds, there is also increasing evidence of geographical differentiation in sperm traits within species, especially in those with strong sperm competition. Geographical divergences in sperm traits might imply the formation of a reproductive barrier in a speciation process. Here we study sperm morphology variation of willow warblers Phylloscopus trochilus in a geographical context in Scandinavia, across the range of two subspecies that are differentiated in certain genetic markers, morphology and migratory routes. We describe geographical patterns in genotypes (two previously described single‐nucleotide polymorphism (SNP) markers and four polymorphic microsatellites); in wing length, tarsus length and body mass; and in sperm traits of 330 male willow warblers sampled at 33 localities across Norway (58°N–69°N). Birds were on average larger and longer‐winged in the north (spp. acredula) than in the south (spp. trochilus), and showed a sigmoid change in the SNP allele frequencies and body morphology around 65°N. We found no evidence of genetic structuring in the microsatellites. There was no geographical variation in sperm traits across Norway, except that sperm heads were on average longer in the south. Sperm head length was also associated with the two SNP markers, with longer sperm heads for the southern alleles, and midpiece length was weakly associated with one of the SNP markers. Similar among‐male variances in total sperm length among the 33 sampling sites indicate uniform levels of sperm competition across Norway. We conclude that sperm morphology remains a rather undifferentiated trait between the two willow warbler subspecies in Scandinavia, which is consistent with a pattern of a shallow genetic divergence. This indicates that sperm morphology is not a reproductive barrier maintaining the narrow hybrid zone.     

Haematocrit values in chinstrap penguins (Pygoscelis antarctica): variation with age and reproductive status

We studied haematocrit variation with age and reproductive status in chinstrap penguins (Pygoscelis antarctica) at Deception Island, South Shetlands. There was a significant difference in haematocrit values between different age groups (adults vs nestlings and juveniles). Adults had a higher haematocrit value than juveniles and nestlings and juveniles had higher haematocrit values than nestlings. There was also a significant difference in haematocrit values between adults of different status (breeding vs moulting). Breeding adults had higher haematocrit values than moulting adults. The observed differences between age groups were thought to be the result of different oxygen demands due to exercise. The observed differences between adults of different status could reflect the different demands associated with reproduction and moult fasting.     

The effects of host plant on the coccinellid functional response: Is the conifer specialist Aphidecta obliterata (L.) (Coleoptera: Coccinellidae) better adapted to spruce than the generalist Adalia bipunctata (L.) (Coleoptera: Coccinellidae)?

Female Aphidecta obliterata consumed an average of 26.6 ± 5.8 Elatobium abietinum and males consumed an average of 19.9 ± 3.2 Elatobium abietinum, but there was no significant difference in the number consumed between the sexes. In Petri dish trials, the larval stages of A. obliterata and all stages of Adalia bipunctata appeared to exhibit a Type II response to prey density, although A. obliterata adults showed a more linear response to prey density. There was no significant difference between the functional responses of the 3rd instars of the two coccinellid species, but there was a significant difference between the responses shown by the 1st instars, with A. obliterata larvae consuming more than those of A. bipunctata, especially at low densities, suggesting that the two species respond differently to an increase in prey density.At low prey densities, adults and 4th instars of both species exhibited a similar response to an increase in prey density but at higher densities the 4th instars and adult stages of A. bipunctata showed higher attack rates when compared with the same stages of A. obliterata.Adult and 4th instar A. obliterata exhibited Type II functional responses on spruce sections. The 4th instar A. obliterata larva appeared to be a more effective predator than the adult of the species, and was more effective when compared with adult A. bipunctata at lower prey densities but A. bipunctata adults appeared to be a more effective predator at higher prey densities.The host plant affected the rate of consumption by adult A. obliterata as adults on Sitka spruce sections consumed significantly higher numbers of aphids than their counterparts on Norway spruce. This was most noticeable at densities above 16 aphids.The distributions of the two coccinellid species in the olfactometer were significantly affected by the presence of host plant material. Aphidecta obliterata adults were found in significantly higher numbers in the Sitka spruce chambers than the control chambers (those without plant material). Adalia bipunctata adults were found in significantly lower numbers in the Norway spruce chamber than the control chamber.Although A. bipunctata has a higher level of voracity than A. obliterata, the latter is more adapted to the spruce environment and the boom and bust population dynamics of E. abietinum.     

Density-dependent variation in body mass of voles

We studied inter-annual, spatial and sexual variation in the body mass of bank volesMyodes glareolus Schreber, 1780 and grey-sided volesMyodes rufocanus Sundevall, 1846 using live trappings from two grids on the southand north-facing slopes of a mountain valley in Southern Norway. Variation in spring density of the four populations was consistent with cyclic dynamics (n=7,s-values >0.5). Individuals caught on the south-facing slope were larger than those caught on the north-facing slope. Reproductively mature bank vole males were smaller than females, whereas reproductively mature grey-sided vole males were larger than females. Body mass was related to density in both species. In bank voles, we found a direct positive density dependence caused by a higher rate of survival at higher densities resulting from individual allocation of resources from reproduction to survival and growth. In grey-sided voles, we found a negative delayed density dependence resulting from grazing on preferred plants that determined the resource available for individual vole growth the following year.     

Comparing Alkaline Phosphatase PhoX-Encoding Genes in Two Contrasting Habitats of the Large Eutrophic Lake Taihu,China

River mouth areas of eutrophic lakes have frequent cyanobacterial blooms and high organophosphate mineralization rates. To determine the genetic background of the high organophosphate mineralization rate in river mouth areas of eutrophic lakes, we compared the diversity and abundance of bacterial alkaline phosphatase PhoX-encoding genes in the river mouth areas and the lake center within the large eutrophic Lake Taihu, China. Results showed that the seasonal average values of phosphorus content, bacterial alkaline phosphatase activity, diversity, and abundance of bacterial phoX in the river mouths were significantly higher than those in the lake center, and 47% of all identified genotypes were present only in the river mouths. The genotypes most similar to alphaproteobacterial phoX dominated in both regions, with 72% and 64% of the total genotypes in the lake center and river mouths, respectively. More than 40% of the total phoX genotypes were only found in the river mouths, however, indicating the special microbial nature in this hypereutrophic habitat. The differences in contents of dissolved organic carbon and phosphorus were related to the divergences of bacterial phoX genotypes in these two contrasting habitats. Because some dominant genotypes most similar to phoX genes of alphaproteobacterial genus were observed only in the river mouth areas, such as Microvirga sp. WSM3557, and because the hydraulic retention time of Lake Taihu is insufficient to shape the special bacterial groups in the river mouths, allochthonous inputs of nutrients and bacterial species could make contributions to explain the different organophosphate mineralization rates in these two contrasting habitats of the large eutrophic lake.     

HindIII DNA polymorphism in the lipoprotein lipase gene and plasma lipid phenotypes and carotid artery atherosclerosis

Lipoprotein lipase (LPL) is the rate limiting enzyme in the hydrolysis of core triglyceride in chylomicron and very low density lipoprotein (VLDL) thus affecting a broad spectrum of plasma lipid levels. In this paper, we investigated the association of a HindIII polymorphism in the LPL gene with plasma lipid levels and carotid artery wall thickness measured by B-mode ultrasonography. A total of 238 Caucasian subjects were selected from the Atherosclerosis Risk In Community (ARIC) study (male = 131, female = 107) based on their fasting triglyceride and LDL-cholesterol levels: normolipidemic (n = 48), hypertriglyceridemic (n = 44), hypercholesterolemic (n = 36), and hypertriglyceridemic-hypercholesterolemic (n = 110) groups. We observed a marginally significant association between lipid phenotypes and HindIII genotypes (P = 0.04) in males, with the hypertriglyceridemic and hypercholesterolemic groups having a higher frequency (0.65) of the H+H+ genotype than the other two groups (pooled: 0.55). In males, there was also a significant association between HindIII genotypes and carotid artery wall thickness after considering the effects of age, body mass index, cigarette smoking, lipid phenotype and diabetes status (P = 0.013), with the H+H+ genotype having a higher average value of carotid artery wall thickness (0.84 ± 0.15 mm) than the other two genotype groups (0.76 ± 0.14 mm in H+H– genotype class, 0.75 ± 0.13 mm in H–H– genotype class). In females, no significant associations among LPL HindIII genotype, lipid phenotype and carotid artery wall thickness were observed. These results suggest that the LPL HindIII polymorphism influences LPL-catalyzed, triglyceride-rich lipoprotein metabolism and carotid artery atherosclerosis in a gender-specific manner. Received: 28 December 1995 / Revised: 2 May 1996     

Minimal gene flow from introduced brown trout (Salmo trutta L.) after 30 years of stocking

Lake Tinnsjø, Norway, has been heavily stocked over three decades with two different brown trout stocks. A population genetic survey based on 13 allozyme coding loci was conducted to investigate the genetic composition of the present trout population and the genetic impact of the two donors. Contrary to expectations, highly significant differences in allelic frequencies between Lake Tinnsjø trout and the two donor stocks were found at several loci, suggesting minimal gene flow from stocked trout to the wild populations. Pairwise genetic distance values were several times higher between donor stocks and Lake Tinnsjø samples than between the various spawning inlets to Lake Tinnsjø. F_st increased from 0.024 to 0.090, when samples from the donor stocks were included in the material. There were no deviations from expected Hardy–Weinberg distribution of genotypes in the spawning inlets. It was concluded that the donor stocks contributed little to the trout gene pool in the lake. A possible exception is the trout population below the lake outlet.     

Accuracy of estimated breeding values with genomic information on males,females, or both: an example on broiler chicken

Background

As more and more genotypes become available, accuracy of genomic evaluations can potentially increase. However, the impact of genotype data on accuracy depends on the structure of the genotyped cohort. For populations such as dairy cattle, the greatest benefit has come from genotyping sires with high accuracy, whereas the benefit due to adding genotypes from cows was smaller. In broiler chicken breeding programs, males have less progeny than dairy bulls, females have more progeny than dairy cows, and most production traits are recorded for both sexes. Consequently, genotyping both sexes in broiler chickens may be more advantageous than in dairy cattle.

Methods

We studied the contribution of genotypes from males and females using a real dataset with genotypes on 15 723 broiler chickens. Genomic evaluations used three training sets that included only males (4648), only females (8100), and both sexes (12 748). Realized accuracies of genomic estimated breeding values (GEBV) were used to evaluate the benefit of including genotypes for different training populations on genomic predictions of young genotyped chickens.

Results

Using genotypes on males, the average increase in accuracy of GEBV over pedigree-based EBV for males and females was 12 and 1 percentage points, respectively. Using female genotypes, this increase was 1 and 18 percentage points, respectively. Using genotypes of both sexes increased accuracies by 19 points for males and 20 points for females. For two traits with similar heritabilities and amounts of information, realized accuracies from cross-validation were lower for the trait that was under strong selection.

Conclusions

Overall, genotyping males and females improves predictions of all young genotyped chickens, regardless of sex. Therefore, when males and females both contribute to genetic progress of the population, genotyping both sexes may be the best option.     

Hematological and plasma chemistry of Indian major carp,Labeo rohita (Hamilton, 1822)

Hematological and plasma chemistry indices are simple and essential diagnostic tools for monitoring the physiological and health status of fish. Aim of the present study was to obtain reference values for the hematological and plasma chemistry of wild populations of Labeo rohita captured in a freshwater pond between July 2008 and June 2010. These reference values and the mean were evaluated according to sex and season. In summer, the red blood cells (1.84 × 106 38 per cubic mm), haemoglobin (8.52 gm dl^?1) and haematocrit (31.49%) were highest in males, whereas the maximum values for white blood cells (5.635 × 103 40 per cubic mm) were found in females, however, no significant variation of mean corpuscular volume (MCV) or mean corpuscular haemoglobin (MCH) was observed between sexes. Various blood parameter levels between the sexes in summer were notably different from those measured in other seasons except for MCH and MCHC values (p < 0.05). Compared to most teleosts, the L. rohita has similar mean values for PCV and Hb. Throughout summer the glucose (76.0 mg dl^?1), lipid (3.41 gm dl^?1) and cholesterol (145.0 mg dl^?1) levels were highest. In spring the plasma protein levels were higher in males, but higher in winter for females. Consequently, the seasons are key factors when using blood parameters as biomarkers for environmental alterations.     

Interactions between green spruce aphid (Elatobium abietinum (Walker)) and Norway and Sitka spruce under high and low nutrient conditions

1 Green spruce aphid (Elatobium abietinum) is a serious pest of spruce (Picea spp.) in north‐west Europe, causing defoliation of one‐year‐old and older needles. 2 Relationships between population development of E. abietinum, needle loss and tree growth were compared for five pure genotypes of Sitka spruce and mixed‐genotype material of Sitka and Norway spruce, grown under high and low nutrient conditions. 3 Despite wide differences in flushing date between spruce genotypes, E. abietinum populations peaked on the same date on each genotype and on the mixed‐genotype material, irrespective of nutrient supply. 4 Larger aphid populations developed on trees grown under high nutrient conditions than under low nutrients. However, more needles were lost per aphid in the low nutrient treatment and overall defoliation rates in the two nutrient treatments were similar. 5 Total aphid numbers differed significantly between Sitka spruce genotypes within nutrient treatments, but not in relation to bud‐burst or needle terpene content. Reductions in height growth caused by infestation were greater (15–44%), and related to mean aphid densities and defoliation, in the low nutrient treatment, but were smaller (11–27%) and not related to aphid density and defoliation in the high nutrient treatment. 6 Development of E. abietinum populations was similar on Norway and Sitka spruce, but Norway spruce lost fewer needles. However, the effects of infestation on tree growth were more closely related to aphid density and were similar for Norway and Sitka spruce. 7 Infestation caused a decrease in total root dry weight of Norway and Sitka spruce in proportion to the reductions observed in above‐ground growth.     

Leaf development and phenology of Triticum aestivum and T. durum under different moisture regimes

Wheat grain yield production in the rain-fed areas is limited by water deficits during crop growth. A greenhouse experiment was conducted during spring 1992 at ICARDA, Tel Hadya, Syria, with eight genotypes representing two Triticum species (Triticum turgidum var. durum and Triticum aestivum L.) under four soil-moisture regimes (95%, 75%, 55%, and 35% field capacity) to study the effect of water deficit on leaf development. The phyllochron was similar in the two species across the watering regimes. The range in variation in phyllochron among the genotypes was similar in the two species. Phyllochron response to water stress among genotypes was distinct in the driest regime in both species. Cham 6 (T. aestivum) and Gallareta (T. turgidum var. durum) had similar phyllochron across all moisture regimes whereas in other genotypes phyllochron was higher in the dries regime. Leaf area decreased with increasing moisture stress. Triticum turgidum var. durum genotypes were later in flowering as they had, on average, one leaf more than Triticum aestivum genotypes with similar leaf appearance rates.     

DNA nucleotide sequence homologies between some zoosporic fungi

Summary Representative species from the monoflagellate Blastocladiales and the biflagellate Saprolegniales were studied for their DNA base composition, heterogeneity, nucleotide sequence homology and divergence. Intergeneric, intrageneric and interstrain DNAs of Blastocladiales were heterogenous. The G+C values for their main component (average 64 percent) and two minor ones (average 52 and 44 percent) were found to be significantly higher than the corresponding values from the biflagellate Saprolegnia ferax (55, 46 and 36 percent respectively). In Allomyces species, the two hybrid, male and female strains were found to have closer homology with their parental types than these last between themselves. Among Blastocladiales, interspecific similarities between the epigynous A. macrogynus and the hypogynous A. arbuscula were higher (average 75 percent) than intergeneric similarities between Allomyces and Blastocladiella (average 58 percent). The biflagellate S. ferax was found to be distantly related to the uniflagellate Allomyces (average 48 percent similarity). The nucleotide sequence divergences obtained from thermal elution data correlated the hybridization values.     

Chemical characteristics and lipid fraction quality of sardines (Sardina pilchardus W.): influence of sex and length

An experimental investigation was carried out on sardines (Sardina pilchardus W.) with the aim to evaluate the influence of sex and total length (TL) on total weight (TW) and fillet weight (FW), chemical composition, and lipid fraction quality. Special emphasis was given with respect to the content of polyunsaturated fatty acids (PUFA), which are considered important from a dietary, nutritional, and functional point of view. Sardines were subdivided as a function of TL and sex. Four groups were obtained: (i) males, TL 130–155 mm, (ii) males, TL 155–180 mm, (iii) females, TL 130–155 mm, (iv) females, TL 155–180 mm. Fatty acid composition and chemical analyses were subsequently carried out on the sardine fillets; results showed higher TW and FW in female sardines and higher percentage value of FW/TW ratio in male sardines, indicating a higher incidence of viscera and other non‐edible parts in females than in males. Moreover, total lipids were on average significantly higher (P < 0.05) in longer specimens (TL = 155–180 mm) than in shorter ones (TL = 130–155 mm). Finally, total saturated fatty acids (SFA) and total mono‐unsaturated fatty acids (MUFA) values were found to be significantly higher in females than in males; such values increased significantly (P < 0.01) only for total SFA from shorter to longer sardines. As for total PUFA, both ω6 and ω3 were found to be higher in male than in female sardines and significantly higher (P < 0.001) in larger than in smaller sizes. Total PUFA/total SFA ratio were in all cases close to 1, with higher values in male than in female sardines and in larger than in smaller specimens.     

Smolting characters in anadromous and resident Arctic charr, Salvelinus alpinus (L.)

Morphological differences, haematocrit value and chloride cells were examined in downstream migrating Arctic charr, Salvelinus alpinus (L.) from the Hals River, North Norway, and resident charr from Lake Storvann. Fish were classified as visual parr, silvery parr or smolt based on the degree of silvering and the Occurrence of lateral parr marks. On average, 47% of downstream migrating Arctic charr were classified as visual molts, but only 14% of the resident charr. Charr longer than 20 cm fork length were mainly classified as visual molts. Morphometrical analyses of body size and shape revealed that most of the variations could be explained in terms of variations in fork length. Length adjusted ratios of post-anal distances were significantly higher in migrating charr than in resident charr. The number of developed chloride cells, the cell nucleus diameter in the basin of secondary gill lamellae and blood haematocrit values were significantly higher among downstream migrating charr than in resident charr. However, morphological differences between anadromous and resident Arctic charr were not necessarily synchronized with the development of chloride cells.     

Experimental analysis of the evolutionary potential of hybridization in leopard frogs (Anura: Ranidae)

Artificial crossing using Rana blairi and R. sphenocephala frogs produced conspecific, interspecific and F₁ backcross hybrid genotypes. Although hybrid males used in the crosses were sterile, crosses using hybrid females produced viable larvae. The larval performance of resultant parental and hybrid genotypes was measured in experimental ponds at two densities. Density significantly affected survival, body mass at metamorphosis, larval period length and metamorphosis for all genotypes. Survival was the same among genotypes, but decreased with increasing density. Body mass at metamorphosis was the same among genotypes, but decreased with increasing density. Larval period increased with increasing density. Among genotypes, larvae from the conspecific R. sphenocephala cross had the shortest larval period while larvae from the conspecific R. blairi cross had the longest larval period. All hybrid genotypes had larval periods longer than R. sphenocephala, but shorter than R. blairi. The percentage of individuals metamorphosing was highest for R. sphenocephala ponds and lowest for R. blairi ponds across densities. Ponds with hybrid larvae produced a greater proportion of metamorphs than those with R. blairi larvae, but a smaller proportion than R. sphenocephala ponds. Equivalent or increased relative larval performance of hybrid genotypes under the conditions of our experiment suggests that hybrid genotypes may possess similar or higher fitnesses relative to their progenitors in some environments. Reduced fertility of adult hybrid males is a powerful selective force against natural hybridization. Nevertheless, because of the successful reproduction by female hybrids, natural hybridization has the potential to serve as a mechanism for the introgression of novel genetic variation that can benefit both R. blairi and R. sphenocephala in fluctuating and unpredictable larval environments. Experimental determination of the fitness of parental and hybrid genotypes is crucial for a comprehensive understanding of the effects of hybridization on organismal evolution.