The major histocompatibility complex (MHC) has become a paradigm for how selection can act to maintain adaptively important genetic diversity in natural populations. Here, we review the contribution of studies on the MHC in non-model species to our understanding of how selection affects MHC diversity, emphasising how ecological and ethological processes influence the tempo and mode of evolution at the MHC, and conversely, how variability at the MHC affects individual fitness, population dynamics and viability. We focus on three main areas: the types of information that have been used to detect the action of selection on MHC genes; the relative contributions of parasite-mediated and sexual selection on the maintenance of MHC diversity; and possible future lines of research that may help resolve some of the unanswered issues associated with MHC evolution.

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The evolutionary ecology of the major histocompatibility complex.

In less than half a century, molecular markers have totally changed our view of nature, and in the process they have evolved themselves. However, all of the molecular methods developed over the years to detect variation do so in one of only three conceptually different classes of marker: protein variants (allozymes), DNA sequence polymorphism and DNA repeat variation. The latest techniques promise to provide cheap, high-throughput methods for genotyping existing markers, but might other traditional approaches offer better value for some applications?

The evolution of molecular markers — just a matter of fashion?

The concept of a sustainable yield (SY, Gulland, 1983; Lannan et al., 1989) has dominated fisheries management for almost 50 years. The central idea is that each stock has a harvestable surplus, and that fisheries that do not exceed this will not compromise the stock’s natural perpetuation. A basic assumption is that the fishery targets a unit stock with definable patterns of recruitment and mortality. Although it is difficult to reach agreement on what constitutes a stock (Gauldie, 1991), the notion of population units with varying degrees of temporal or spatial integrity stimulated a quest to characterize and identify such assemblages. It became apparent that few species form single homogeneous populations, but rather that fish species are often composed of discrete stocks, and that these stocks may react to harvesting more or less independently. It was the idea of independent responses of different stocks to exploitation that demanded information on stock structure. Hence, the stock concept was linked strongly, at least in theory, with the desire to balance the impacts of harvesting with efforts to ensure continued economic returns. What was much more difficult to achieve than the theory was: (i) how to actually recognize a stock, and (ii) how to translate stock structure data into fishery practices.

Molecular genetics and the stock concept in fisheries

In the 1960s, when population geneticists first began to collect data on the amount of genetic variation in natural populations, balancing selection was invoked as a possible explanation for how such high levels of molecular variation are maintained. However, the predictions of the neutral theory of molecular evolution have since become the standard by which cases of balancing selection may be inferred. Here we review the evidence for balancing selection acting on the major histocompatibility complex (MHC) of vertebrates, a genetic system that defies many of the predictions of neutrality. We apply many widely used tests of neutrality to MHC data as a benchmark for assessing the power of these tests. These tests can be categorized as detecting selection in the current generation, over the history of populations, or over the histories of species. We find that selection is not detectable in MHC datasets in every generation, population, or every evolutionary lineage. This suggests either that selection on the MHC is heterogeneous or that many of the current neutrality tests lack sufficient power to detect the selection consistently. Additionally, we identify a potential inference problem associated with several tests of neutrality. We demonstrate that the signals of selection may be generated in a relatively short period of microevolutionary time, yet these signals may take exceptionally long periods of time to be erased in the absence of selection. This is especially true for the neutrality test based on the ratio of nonsynonymous to synonymous substitutions. Inference of the nature of the selection events that create such signals should be approached with caution. However, a combination of tests on different time scales may overcome such problems.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.0014-3820.2003.tb00580.x

Perspective: detecting adaptive molecular polymorphism: lessons from the MHC.

The Diptera are the second most important order among flower-visiting (anthophilous) and flower-pollinating insects worldwide. Their taxonomic diversity ranges from Nematocera to Brachycera, including most families within the suborders. Especially important are Syrphidae, Bombyliidae, and Muscoidea. Other families, especially of small flies, are less appreciated and often overlooked for their associations with flowers. We have compiled records of their flower visitations to show that they may be more prevalent than usually thought. Our knowledge of anthophilous Diptera needs to be enhanced by future research concerning (i) the significance of nocturnal Nematocera and acalypterate muscoids as pollinators, (ii) the extent to which the relatively ineffective pollen-carrying ability of some taxa can be compensated by the abundance of individuals, and (iii) the role of Diptera as pollinators of the first flowering plants (Angiospermae) by using phylogenetic and palaeontological evidence. Specializations in floral relationships involve the morphology of Diptera, especially of their mouthparts, nutritional requirements, and behaviour, as well as concomitant floral attributes. The South African flora has the most highly specialized relations with dipterous pollinators, but in arctic and alpine generalist fly–flower relations are important in pollination and fly nutrition.

Flies and flowers: taxonomic diversity of anthophiles and pollinators

Forty-one (Mangifera indica L.) cultivars were characterized electrophoretically using the isozyme systems aconitase, isocitrate dehydrogenase, leucine aminopeptidase, phosphoglucose isomerase, phosphoglucomutase, and triosephosphate isomerase. The outcross origin of some of the mango cultivars was supported by the isozymic banding patterns. Reported parentage of some other cultivars was not consistent with their isozymic banding patterns.

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Enzyme Polymorphism in Mango

Outcrossing rates in 'Hass' and Fuerte' avocado (Persea americana Mill.) blocks were determined during fruit development using the isozyme systems of malate dehydrogenase (MDH, EC 1.1.1.37), leucine aminopeptidase (LAP, EC 3.4.11.1), and triosephosphate isomerase (TPI, EC 5.3.1.1) as genetic markers. Abscission of avocado fruitlets was selective and was greatly influenced by the pollen parent. Thus, in 'Hass' trees subjected to cross-pollination by 'Ettinger' and 'Fuerte', the population of 'Hass' fruitlets 1 month after fruit set consisted mainly of 'Hass' selfs; however, during fruit development, the rate of 'Hass' selfs decreased and the rate of 'Hass' hybrid fruitlets produced by 'Ettinger' and 'Fuerte' increased. By the end of fruit abscission, the surviving mature 'Hass' fruits were mostly 'Ettinger' hybrids. The 'Hass' fruit yield was found to correlate significantly with the rate of Outcrossing with 'Ettinger'. When 'Ettinger' served as a pollen parent for 'Fuerte', the Outcrossing rate in trees adjacent to 'Ettinger' was about 40%, which shows that cross-pollination among avocado cultivars of the same flowering group can be substantial at close proximity. The Outcrossing rate in 'Fuerte' diminished with increasing distance from 'Ettinger', but no effect on yield was observed.

Pollen Parent Effect on Outcrossing Rate in 'Hass' and 'Fuerte' Avocado Plots during Fruit Development

Using a malate dehydrogenase isozyme system, it was possible to identify the pollen donor parent in progenies of avocado (Persea americana Mill.) obtained by enclosing 2 cultivars of complementary flower type in a screenhouse with a beehive. The observed percentage of cross-pollination ranged from 7% to 92%. Isozymes are demonstrated to be a practical way of identifying hybrids.

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Selfed and crossed proportions of avocado progenies produced by caged pairs of complementary cultivars

Fruit produced by adjacent blocks of 'Mauritius' and 'Floridian' lychee (Litchi chinensis Sonn.) were sampled at four different stages of development and the embryos were analyzed for pollen parentage by phosphoglucos e isomerase (PGI; EC 5.3.1.9) isozyme system. Hybrid percentage increased significantly from ≈5 weeks after fruit set to maturity as follows: from 29.5% to 76.3% in 'Mauritius' and from 74.2% to 92.5% in 'Floridian'. These findings clearly indicate selective abscission of selfed fruitlets. In 'Mauritius', yield was not related to the distance from the pollenizer block or hybrid percentage. In 'Floridian', yield of trees adjacent to the 'Mauritius' pollenizer was higher by 36% than that of trees at a distance of 24 m. The correlation between 'Flordian' yield and hybrid percentage tended toward significance (r= 0.64, P = 0.08). In addition, in both cultivars, fruit and seed weights were affected by the pollen parent: outcrossed fruit were heavier and contained heavier seeds than selfed ones.

/pdf/pollen-parent-effect-on-the-selective-abscission-of-1ab92cr503.pdf

Pollen Parent Effect on the Selective Abscission of 'Mauritius' and 'Floridian' Lychee Fruitlets

Fruit development and abscission in 'Mauritius' lychee (Litchi chinensis Sonn.) were studied over three consecutive seasons. Each season, two distinct abscission periods were observed. The first started at the end of full female bloom and continued for ≈ 4 weeks. Of the initial number of female flowers, 85 % to 90 % abscised during this period. The second period began after a lag period of ≈ ≈ 1 week and lasted ≈ 2 weeks. About half of the remaining fruitlets abscised during this wave. AU of these fruitlets contained an embryo. The second wave coincided with a period of rapid embryo growth and endosperm loss. Tipimon (a commercial product containing the triethanolamine salt of the synthetic auxin 2,4,5-TP) consistently and significantly increased marketable fruit yield when applied between the two abscission periods. Chemical name used: 2,4,5 -trichlorophenoxy propionic acid (2,4,5 -TP).

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S. Gazit

Papers

Enzyme Polymorphism in Mango

Pollen Parent Effect on Outcrossing Rate in 'Hass' and 'Fuerte' Avocado Plots during Fruit Development

Selfed and crossed proportions of avocado progenies produced by caged pairs of complementary cultivars

Pollen Parent Effect on the Selective Abscission of 'Mauritius' and 'Floridian' Lychee Fruitlets

`Mauritius' Lychee Fruit Development and Reduced Abscission after Treatment with the Auxin 2,4,5-TP