Genetic divergence over small geographic scales and conservation implications for Commerson's dolphins (Cephalorhynchus commersonii) in southern Argentina

Cephalorhynchus commersonii is distributed in the nearshore coastal waters of South America, and thus is particularly vulnerable to bycatch in coastal nets and trawls. Our study documents genetic structure in presumed Commerson's dolphin subpopulations along the southern Argentina coastline, from the Ría Deseado in the north to Ría Gallegos in the south, and focuses on the potential for depletion in the apparently more heavily impacted Ría Gallegos area. Only two control region (423 bp) haplotypes were shared among all these locations (out of 11 identified), and striking differences in haplotype frequencies between areas are apparent. AMOVA analysis, using mitochondrial sequence data, indicates significant population subdivision (overall F_ST= 0.21, P < 0.001) between Ría Deseado (n= 8), Bahía San Julián (n= 11), Ría Gallegos (n= 31), and a small sample of dolphins from the captive colony at San Diego Seaworld (n= 7) derived from animals originally captured in the Strait of Magellan. Comparisons based on haplotypic distances indicated relatively strong differences between regions (Φ_ST= 0.30, P < 0.001). This research provides the first indication of reduced gene flow and genetic differentiation within local subpopulations of Commerson's dolphins, along a relatively small stretch of coastline.     

Genomic signatures of rapid adaptive evolution in the bluespotted cornetfish,a Mediterranean Lessepsian invader

Biological invasions are increasingly creating ecological and economical problems both on land and in aquatic environments. For over a century, the Mediterranean Sea has steadily been invaded by Indian Ocean/Red Sea species (called Lessepsian invaders) via the Suez Canal, with a current estimate of ~450 species. The bluespotted cornetfish, Fistularia commersonii, considered a ‘Lessepsian sprinter’, entered the Mediterranean in 2000 and by 2007 had spread through the entire basin from Israel to Spain. The situation is unique and interesting both because of its unprecedented rapidity and by the fact that it took this species c. 130 years to immigrate into the Mediterranean. Using genome scans, with restriction site‐associated DNA (RAD) sequencing, we evaluated neutral and selected genomic regions for Mediterranean vs. Red Sea cornetfish individuals. We found that few fixed neutral changes were detectable among populations. However, almost half of the genes associated with the 47 outlier loci (potentially under selection) were related to disease resistance and osmoregulation. Due to the short time elapsed from the beginning of the invasion to our sampling, we interpret these changes as signatures of rapid adaptation that may be explained by several mechanisms including preadaptation and strong local selection. Such genomic regions are therefore good candidates to further study their role in invasion success.     

Post-zygotic gametic selection due to endosperm balance number explains unusual chromosome numbers of 3×× 2× progeny in Solanum

 Crosses between triploid and diploid genotypes are usually the best sources of trisomics in potato as well as in several other crop species. However, 3×× 2× crosses between triploid (2n=3×=36; 2EBN) Solanum commersonii-S. tuberosum hybrids and diploid (2n= 2×=24; 2EBN) genotypes gave progenies with a high number of extra chromosomes, 29–36, suggesting that only eggs with 17–24 chromosomes produced embryos that reached full development. Our hypothesis is that although triploids produce eggs with a range of chromosome numbers, 3×× 2× crosses involving a 2×(2EBN) parent favor eggs with a high chromosome number. These eggs have higher probabilities of possessing the same endosperm balance number (EBN) value (i.e. 1) of gametes produced by the 2EBN diploid parent to give the required 2:1 maternal to paternal EBN ratio in the hybrid endosperm. Under this model, trisomics are produced only if the diploid parent has an EBN of 1. Based on our results and those reported in the literature, it is proposed that in 3×(2EBN) × 2×(2EBN) crosses the endosperm balance number exercises negative selection for gametes with a low chromosome number, and a corresponding low EBN, and positive selection for gametes with a high chromosome number and EBN. Received: 2 April 1998 / Revision accepted: 27 October 1998     

First record of the bluespotted cornetfish from the Mediterranean Sea

Three specimens of the Indo-Pacific Bluespotted cornetfish Fistularia commersonii are recorded for the first time from the Mediterranean. The presence of this species in the Mediterranean is due to migration from the Red Sea via the Suez Canal.     

Lessepsian fish migration: genetic bottlenecks and parasitological evidence

As a rule, non‐indigenous species (NIS) populations derived from biological invasion events represent a subset of the genetic diversity of the source population. In biological invasions, host–parasite interactions play an important role, and parasitological data for NIS populations can provide useful information such as their area of origin, mechanism of invasion and prospects of success in the new habitat. When both genetic and parasitological data are available, and they suggest the same scenario, the history of an invasion can be inferred with no discrepancy, but when data cannot be reconciled an alternative model should be considered. In this study a comparison of genetic and parasitological data for the Lessepsian migrant the bluespotted cornetfish, Fistularia commersonii, in the Mediterranean Sea presents the opportunity to evaluate the compatibility of information of this nature, and to propose possible invasion scenarios consistent with evidence provided by both criteria.     

Evidence for tetrasomic inheritance in a tetraploid Solanum commersonii (+) S. tuberosum somatic hybrid through the use of molecular markers

In order to assess the potential for interspecific recombination between the cultivated Solanum tuberosum (tbr) and the sexually isolated wild species Solanum commersonii (cmm), genetic analysis of a F₂ progeny obtained by selfing one tetraploid cmm (+) tbr somatic hybrid was performed through molecular markers. For this purpose, the extent of disomic and/or tetrasomic inheritance of species-specific RAPD and AFLP markers was determined by following their segregation in a 90-genotype progeny, and testing all the possible segregation ratios in a selfed tetraploid progeny. The RAPD analysis performed using 16 primers revealed that the cmm-specific RAPDs were mainly (93.7%) duplex markers and were equally distributed between loci with a disomic (46.7%) and tetrasomic (53.3%) inheritance. The AFLP analysis led to the identification of 272 (58%) informative AFLPs, which were either cmm- or tbr-specific markers. About 63% of cmm-specific AFLPs were duplex loci, most of which (92.6%) were inherited as tetrasomic loci. As regards the tbr-specific AFLPs, the percentage of simplex loci (52.9%) was higher than that of duplex loci (32.6%), and among the latter most (88.5%) were inherited as tetrasomic loci. Overall, 130 duplex markers were found, of which 53.1% were cmm-specific and 46.9% were tbr-specific. Out of 130 markers, 18 (13.8%) were inherited as disomic, and 112 (86.2%) as tetrasomic, loci. This implies that the majority of duplex markers were located on chromosomes which at meiosis tend to randomly pair as bivalents or to form tetravalents. The total number of simplex loci was 119, and most of them (82.3%) were tbr-specific loci. In some cases the observed segregation ratios even allowed us to clearly determine whether a random chromosome or chromatid segregation was detected. This was the case of three cmm-specific RAPDs, 19 cmm- and 25 tbr-specific AFLPs, which fit a 20.8:1 or 2.5:1 ratio, both cases for which a clear random chromatid segregation can be assumed, since they represent the limit of segregation expected when the distance between the locus and the centromere always leads to a cross-over event. The percentage of ascertained crossing-over events was around 37% out of the tetrasomically inherited loci clearly identified (128 loci), a value indicating that the flow of genes from the sexually isolated S. commersonii to the cultivated potato is possible, for at least a large proportion of genes. Received: 23 July 2001 / Accepted: 9 August 2001     

Chilling sensitivity of the frost-tolerant potato Solanum commersonii

Frost tolerance has been reported in the shoots of wild, tuberiferous potato species such as Solanum commersonii when the plants are grown in either field or controlled conditions. However, these plants can survive as underground tubers and avoid unfavorable environmental conditions altogether. As such, leaf growth and photosynthesis at low temperature may not be required for survival of the plants. In order to determine the temperature sensitivity of S. commersonii shoots, we examined leaf growth, development and photosynthesis in plants raised at 20/16°C (day/night). 12/9°C and 5/2°C. S. commersonii leaves grown at 5°C exhibited a marked decrease in leaf area and in total chlorophyll (Chl) content per leaf area when compared with leaves grown at 20°C. Furthermore, leaves grown at 5°C did not exhibit the expected decrease in either water content or susceptibility to low-temperature-induced photoinhibition that normally characterizes cold acclimation in frost-tolerant plants. Measurements of CO₂-saturated O₂ evolution showed that the photosynthetic apparatus of 5°C plants was functional, even though the efficiency of photosystem II photochemistry was reduced by growth at 5°C. A decrease in the resolution of the M-peak in the slow transients for Chl a fluorescence in leaves grown at 12 and 5°C and in all leaves exposed to high light at 5°C indicated that low temperature significantly affected processes on the reducing side of Q_A, the primary quinone electron acceptor in photosystem II. Thus S. commarsonii exhibits the characteristics of a plant that is limited by chilling temperatures. Although S. commersonii can tolerate light frosts, its sensitivity to chilling temperatures may result in shoot dieback in winter in its native habitat. The plants may avoid both chilling and freezing temperatures by overwintering as underground tubers.     

Alterations of gene expression in potato (Solanum commersonii) during cold acclimation

Plantlets of Solanum commersonii stem-culture were acclimated at 5°C day/night temperature for 14 days. Cold hardiness increased from – 3.5°C to – 8.6°C. During the course of acclimation, the synthesis of polypeptides was investigated and poly (A⁺) RNA was isolated. Translation products of poly(A⁺) RNA in a rabbit rcticulocyte lysate system were then analyzed. During the 14 days of acclimation, 23 cold-induced polypeptides were identified. Most of them disappeared following 1 day of de-acclimation at a 20/15°C day/night regime. The synthesis of one group of polypeptides is prominent and stable throughout the acclimation period. The other group is transient. The most prominent and stable polypeptides have molecular weights of 21, 22, 31 and 83 kDa.
Acclimation alters translatable mRNA population during the development of cold hardiness. Two mRNAs encoding in vitro translation products at 26 and 27 kDa were identified during the course of acclimation. These proteins may play important roles in the overall programming for the development of cold hardiness in tuber-bearing S. commersonii.     

Freezing tolerance, cold acclimation and oxidative stress in potato. Paraquat tolerance is related to acclimation but is a poor indicator of freezing tolerance

Potato is a species commonly cultivated in temperate areas where the growing season may be interrupted by frosts, resulting in loss of yield. Cultivated potato, Solanum tuberosum, is freezing sensitive, but it has several freezing-tolerant wild potato relatives, one of which is S. commersonii. Our study was aimed to resolve the relationship between enhanced freezing tolerance, acclimation capacity and capacity to tolerate active oxygen species. To be able to characterize freezing tolerant ideotypes, a potato population (S1), which segregates in freezing tolerance, acclimation capacity and capacity to tolerate superoxide radicals, was produced by selfing a somatic hybrid between a freezing-tolerant Solanum commersonii (LT₅₀=-4.6°C) and -sensitive S. tuberosum (LT₅₀=-3.0°C). The distribution of non-acclimated freezing tolerance (NA-freezing tolerance) of the S1 population varied between the parental lines and we were able to identify genotypes, having significantly high or low NA-freezing tolerance. When a population of 25 genotypes was tested both for NA-freezing and paraquat (PQ) tolerance, no correlation was found between these two traits (R = 0.02). However, the most NA-freezing tolerant genotypes were also among the most PQ tolerant plants. Simultaneously, one of the NA-freezing sensitive genotypes (2022) (LT₅₀=-3.0°C) was observed to be PQ tolerant. These conflicting results may reflect a significant, but not obligatory, role of superoxide scavenging mechanisms in the NA-freezing tolerance of S. commersonii. The freezing tolerance after cold acclimation (CA-freezing tolerance) and the acclimation capacity (AC) was measured after acclimation for 7 days at 4/2°C. Lack of correlation between NA-freezing tolerance and AC (R =-0.05) in the S1 population points to independent genetic control of NA-freezing tolerance and AC in Solanum sp. Increased freezing tolerance after cold acclimation was clearly related to PQ tolerance of all S1 genotypes, especially those having good acclimation capacity. The rapid loss of improved PQ tolerance under deacclimation conditions confirmed the close relationship between the process of cold acclimation and enhanced PQ tolerance. Here, we report an increased PQ tolerance in cold-acclimated plants compared to non-acclimated controls. However, we concluded that high PQ tolerance is not a good indicator of actual freezing tolerance and should not be used as a selectable marker for the identification of a freezing-tolerant genotype.