Delayed population explosion of an introduced butterfly

1. The causes of lagged population and geographical range expansions after species introductions are poorly understood, and there are relatively few detailed case studies. 2. We document the 29-year history of population dynamics and structure for a population of Euphydryas gillettii Barnes that was introduced to the Colorado Rocky Mountains, USA in 1977. 3. The population size remained low (< 200 individuals) and confined to a single habitat patch (approximately 2.25 ha) to 1998. These values are similar to those of many other populations within the natural geographical range of the species. 4. However, by 2002 the population increased dramatically to > 3000 individuals and covered approximately 70 ha, nearly all to the south of the original site. The direction of population expansion was the same as that of predominant winds. 5. By 2004, the butterfly's local distribution had retracted mainly to three habitat patches. It thus exhibited a 'surge/contraction' form of population growth. Searches within 15 km of the original site yielded no other new populations. 6. In 2005, butterfly numbers crashed, but all three habitat patches remained occupied. The populations within each patch did not decrease in the same proportions, suggesting independent dynamics that are characteristic of metapopulations. 7. We postulate that this behaviour results, in this species, in establishment of satellite populations and, given appropriate habitat structure, may result in lagged or punctuated expansions of introduced populations.     

Independent S-Locus Mutations Caused Self-Fertility in Arabidopsis thaliana

A common yet poorly understood evolutionary transition among flowering plants is a switch from outbreeding to an inbreeding mode of mating. The model plant Arabidopsis thaliana evolved to an inbreeding state through the loss of self-incompatibility, a pollen-rejection system in which pollen recognition by the stigma is determined by tightly linked and co-evolving alleles of the S-locus receptor kinase (SRK) and its S-locus cysteine-rich ligand (SCR). Transformation of A. thaliana, with a functional AlSRKb-SCRb gene pair from its outcrossing relative A. lyrata, demonstrated that A. thaliana accessions harbor different sets of cryptic self-fertility–promoting mutations, not only in S-locus genes, but also in other loci required for self-incompatibility. However, it is still not known how many times and in what manner the switch to self-fertility occurred in the A. thaliana lineage. Here, we report on our identification of four accessions that are reverted to full self-incompatibility by transformation with AlSRKb-SCRb, bringing to five the number of accessions in which self-fertility is due to, and was likely caused by, S-locus inactivation. Analysis of S-haplotype organization reveals that inter-haplotypic recombination events, rearrangements, and deletions have restructured the S locus and its genes in these accessions. We also perform a Quantitative Trait Loci (QTL) analysis to identify modifier loci associated with self-fertility in the Col-0 reference accession, which cannot be reverted to full self-incompatibility. Our results indicate that the transition to inbreeding occurred by at least two, and possibly more, independent S-locus mutations, and identify a novel unstable modifier locus that contributes to self-fertility in Col-0.     

Analysis of the membrane-interacting domains of myelin basic protein by hydrophobic photolabeling

Myelin basic protein is a water soluble membrane protein which interacts with acidic lipids through some type of hydrophobic interaction in addition to electrostatic interactions. Here we show that it can be labeled from within the lipid bilayer when bound to acidic lipids with the hydrophobic photolabel 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine (TID) and by two lipid photolabels. The latter included one with the reactive group near the apolar/polar interface and one with the reactive group linked to an acyl chain to position it deeper in the bilayer. The regions of the protein which interact hydrophobically with lipid to the greatest extent were determined by cleaving the TID-labeled myelin basic protein (MBP) with cathepsin D into peptides 1-43, 44-89, and 90-170. All three peptides from lipid-bound protein were labeled much more than peptides from the protein labeled in solution. However, the peptide labeling pattern was similar for both environments. The two peptides in the N-terminal half were labeled similarly and about twice as much as the C-terminal peptide indicating that the N-terminal half interacts hydrophobically with lipid more than the C-terminal half. MBP can be modified post-translationally in vivo, including by deamidation, which may alter its interactions with lipid. However, deamidation had no effect on the TID labeling of MBP or on the labeling pattern of the cathepsin D peptides. The site of deamidation has been reported to be in the C-terminal half, and its lack of effect on hydrophobic interactions of MBP with lipid are consistent with the conclusion that the N-terminal half interacts hydrophobically more than the C-terminal half. Since other studies of the interaction of isolated N-terminal and C-terminal peptides with lipid also indicate that the N-terminal half interacts hydrophobically with lipid more than the C-terminal half, these results from photolabeling of the intact protein suggest that the N-terminal half of the intact protein interacts with lipid in a similar way as the isolated peptide. The similar behavior of the intact protein to that of its isolated peptides suggests that when the purified protein binds to acidic lipids, it is in a conformation which allows both halves of the protein to interact independently with the lipid bilayer. That is, it does not form a hydrophobic domain made up from different parts of the protein.     

An immunodominant epitope of myelin basic protein is an amphipathic alpha-helix

Myelin basic protein is a candidate autoantigen in multiple sclerosis. One of its dominant antigenic epitopes is segment Pro85 to Pro96 (human sequence numbering, corresponding to Pro82 to Pro93 in the mouse). There have been several, contradictory predictions of secondary structure in this region; either beta-sheet, alpha-helix, random coil, or combinations thereof have all been proposed. In this paper, molecular dynamics and site-directed spin labeling in aqueous solution indicate that this segment forms a transient alpha-helix, which is stabilized in 30% trifluoroethanol. When bound to a myelin-like membrane surface, this antigenic segment exhibits a depth profile that is characteristic of an amphipathic alpha-helix, penetrating up to 12 A into the bilayer. The alpha-helix is tilted approximately 9 degrees, and the central lysine is in an ideal snorkeling position for side-chain interaction with the negatively charged phospholipid head groups.     

The evolution of wing color: male mate choice opposes adaptive wing color divergence in Colias butterflies

Abstract Correlated evolution of mate signals and mate preference may be constrained if selection pressures acting on mate preference differ from those acting on mate signals. In particular, opposing selection pressures may act on mate preference and signals when traits have sexual as well as nonsexual functions. In the butterfly Colias philodice eriphyle , divergent selection on wing color across an elevational gradient in response to the thermal environment has led to increasing wing melanization at higher elevations. Wing color is also a long-range signal used by males in mate searching. We conducted experiments to test whether sexual selection on wing melanization via male mate choice acts in the same direction as natural selection on mate signals due to the thermal environment. We performed controlled mate choice experiments in the field over an elevational range of 1500 meters using decoy butterflies with different melanization levels. Also, we obtained a more direct estimate of the relation between wing color and sexual selection by measuring mating success in wild-caught females. Both our experiments showed that wing melanization is an important determinant of female mating success in C. p. eriphyle . However, a lack of elevational variation in male mate preference prevents coevolution of mate signals and mate preference, as males at all elevations prefer less-melanized females. We suggest that this apparently maladaptive mate choice may be maintained by differences in detectability between the morphs or by preservation of species recognition.     

The evolution of wing color in Colias butterflies: heritability,sex linkage,and population divergence

We investigated the genetic background of intraspecific variation in wing color across an elevational gradient in the butterfly Colias philodice eriphyle. The degree of wing melanization was an accelerating function of elevation, and differences in wing melanization persisted in a common environment. Full-sibling analysis and parent-offspring regression yielded consistent, moderate to high heritabilities for the degree of wing melanization. The breeding experiments also demonstrated that wing melanization is strongly sex linked. Because traits that differentiate sister species also tend to be sex linked, our results suggest that the genetic mechanisms underlying intraspecific differences in wing melanization are not fundamentally different from those that have been shown to differentiate sister species.     

Learning cell biology as a team: a project-based approach to upper-division cell biology

To help students develop successful strategies for learning how to learn and communicate complex information in cell biology, we developed a quarter-long cell biology class based on team projects. Each team researches a particular human disease and presents information about the cellular structure or process affected by the disease, the cellular and molecular biology of the disease, and recent research focused on understanding the cellular mechanisms of the disease process. To support effective teamwork and to help students develop collaboration skills useful for their future careers, we provide training in working in small groups. A final poster presentation, held in a public forum, summarizes what students have learned throughout the quarter. Although student satisfaction with the course is similar to that of standard lecture-based classes, a project-based class offers unique benefits to both the student and the instructor.     

Females of the sea urchin Strongylocentrotus purpuratus differ in the structures of their egg jelly sulfated fucans

The egg jelly coats of sea urchins contain sulfated fucans which bind to a sperm surface receptor glycoprotein to initiate the signal transduction events resulting in the sperm acrosome reaction. The acrosome reaction is an ion channel regulated exocytosis which is an obligatory event for sperm binding to, and fusion with, the egg. Approximately 90% of individual females of the sea urchin Strongylocentrotus purpuratus spawned eggs having only one of two possible sulfated fucan electrophoretic isotypes, a slow migrating (sulfated fucan I), or a fast migrating (sulfated fucan II) isotype. The remaining 10% of females spawned eggs having both sulfated fucan isotypes. The two sulfated fucan isotypes were purified from egg jelly coats and their structures determined by NMR spectroscopy and methylation analysis. Both sulfated fucans are linear polysaccharides composed of 1-->3-linked alpha-L-fucopyranosyl units. Sulfated fucan I is entirely sulfated at the O -2 position but with a heterogeneous sulfation pattern at O -4 position. Sulfated fucan II is composed of a regular repeating sequence of 3 residues, as follows: [3-alpha-L-Fuc p - 2,4(OSO3)-1-->3-alpha-L-Fuc p -4(OSO3)-1-->3-alpha-L-Fuc p -4(OSO3)- 1]n. Both purified sulfated fucans have approximately equal potency in inducing the sperm acrosome reaction. The significance of two structurally different sulfated fucans in the egg jelly coat of this species could relate to the finding that the sperm receptor protein which binds sulfated fucan contains two carbohydrate recognition modules of the C-type lectin variety which differ by 50% in their primary structure.      

Lipid phase separation induced by a hydrophobic protein in phosphatidylserine--phosphatidylcholine vesicles

Differential scanning calorimetry (DSC) was used to detect phase separation induced by hydrophobic myelin protein, lipophilin, in a mixture of phosphatidylserine (PS) and dipalmitoylphosphatidylcholine (DPPC). Preferential binding of PS to the boundary layer of lipophilin causes a decrease in the PS content of the remaining lamellar phase with a resultant shift in the phase-transition temperature to a higher temperature. The phase diagram for this mixture in the presence and absence of lipophilin is presented. From the phase diagram, it can be estimated that for an equimolar mixture of PS and DPPC, the boundary layer contains only PS, although for higher DPPC contents, some DPPC can also be found in the boundary layer. In the case where partial phase separation in induced in this mixture by Ca2+ alone, lipophilin increases the phase separation indicating that it also binds PS preferentially in the presence of Ca2+. Preferential binding of two other acidic lipids, phosphatidic acid and phosphatidyl-glycerol, to the boundary layer was also found, including a mixture where the acidic lipid was the higher melting component in the mixture.