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The mating behavior of captive Loligo bleekeri and the paternity of the resulting progeny were examined based on behavioral observations and genetic analyses. In this species, there are 3 mating behaviors (male-parallel, head-to-head, and extra-pair), and 2 sperm storage sites in females (seminal receptacle and the opening of the oviduct), which suggest that sperm competition occurs. All 3 mating behaviors were observed, and females mated often with different males, resulting in multiple paternity within 3 of the 4 broods examined. In each brood, the male to mate last and frequently before the female spawned fertilized the most eggs (87 to 100%). A sneaker male that mated by extra-pair copulation sired 8.5% of the eggs in a brood. Some eggs were fertilized by sperm received before the start of the study, indicating that sperm can be stored for at least several days before a spawning. In the broods with multiple paternity, the paternity patterns differed among egg capsules. Male competition was more intense between similar-sized males than between differentsized males, but body size did not affect the copulative success in the male-parallel position. We found multiple mating and multiple paternity in L. bleekeri. Paternity rates differed depending on the complex of factors, mating position, timing, frequency and duration. Alternative reproductive behaviors would change these factors and lead to different paternity rates.KEY WORDS: Multiple paternity · Reproductive strategy · Squid · Loligo · Microsatellite · Sperm competition Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 298: [219][220][221][222][223][224][225][226][227][228] 2005 just before she spawns . These consort males guard females before and after copulation and while the females spawn, but sometimes large intruders replace the consort males and copulate with the females . Furthermore, extra-pair copulations (EPCs) also occur, in which a small 'sneaker male' copulates with a female that is paired with another male by attaching spermatophores near the female's mouth (Hanlon 1996. The spermatozoa are stored until they are released at spawning, so loliginid females can use spermatozoa from several males when they spawn.When squids spawn, eggs are extruded from the oviduct and pass through the funnel to a position near the mouth. The eggs can be fertilized either as they leave the oviduct, by spermatozoa placed in the mantle cavity during a male-parallel mating, or near the mouth, by spermatozoa stored in the seminal receptacle after a head-to-head mating or EPC. This difference in the fertilization mechanism will presumably affect the reproductive success of each copulative behavior and play a role in sexual selection (Birkhead & Møller 1998).If the reproductive success of each copulative behavior differs, males will compete to mate in the favorable position. Agonistic behavior between males has been shown to escalate through several phases, from visual signaling to physical contact (DiMarco & Ha...

Sympatric speciation has been contentious since its inception, yet is increasingly recognized as important based on accumulating theoretical and empirical support. Here, we present a compelling case of sympatric speciation in a taxon of marine reef fishes using a comparative and mechanistic approach. Hexagrammos otakii and H. agrammus occur in sympatry throughout their ranges. Molecular sequence data from six loci, with complete sampling of the genus, support monophyly of these sister species. Although hybridization occurs frequently with an allopatric congener in an area of slight distributional overlap, we found no F(1) hybrids between the focal sympatric taxa throughout their coextensive ranges. We present genetic evidence for complete reproductive isolation based on SNP analysis of 382 individuals indicating fixed polymorphisms, with no shared haplotypes or genotypes, between sympatric species. To address questions of speciation, we take a mechanistic approach and directly compare aspects of reproductive isolation between allopatric and sympatric taxa both in nature and in the laboratory. We conclude that the buildup of reproductive isolation is strikingly different in sympatric vs. allopatric taxa, consistent with theoretical predictions. Lab reared hybrids from allopatric species crosses exhibit severe fitness effects in the F(1) or backcross generation. No intrinsic fitness effects are observed in F(1) hybrids from sympatric species pairs, however these treatments exhibited reduced fertilization success and complete pre-mating isolation is implied in nature because F(1) hybrid adults do not occur. Our study addresses limitations of previous studies and supports new criteria for inferring sympatric speciation.

Natural hybrids between the boreal species Hexagrammos octogrammus and two temperate species Hexagrammos agrammus and Hexagrammos otakii were observed frequently in southern Hokkaido, Japan. Previous studies revealed that H. octogrammus is a maternal ancestor of both hybrids; the hybrids are all fertile females and they frequently breed with paternal species. Although such rampant hybridization occurs, species boundaries have been maintained in the hybrid zone. Possible explanations for the absence of introgressions, despite the frequent backcrossing, might include clonal reproduction: parthenogenesis, gynogenesis and hybridogenesis. The natural hybrids produced haploid eggs that contained only the H. octogrammus genome (maternal ancestor) with discarded paternal genome and generated F1 -hybrid type offspring by fertilization with the haploid sperm of H. agrammus or H. otakii (paternal ancestor). This reproductive mode was found in an artificial backcross hybrid between the natural hybrid and a male of the paternal ancestor. These findings indicate that the natural hybrids adopt hybridogenesis with high possibility and produce successive generations through hybridogenesis by backcrossing with the paternal ancestor. These hybrids of Hexagrammos represent the first hybridogenetic system found from marine fishes that widely inhabit the North Pacific Ocean. In contrast with other hybridogenetic systems, these Hexagrammos hybrids coexist with all three ancestral species in the hybrid zone. The coexistence mechanism is also discussed.

Divergent natural selection is thought to play a vital role in speciation, but clear, measurable examples from nature are still few. Among the many possible sources of divergent natural selection, predation pressure may be important because predators are ubiquitous in food webs. Here, we show evidence for divergent natural selection in a Lake Tanganyika cichlid, Telmatochromis temporalis, which uses burrows under stones or empty snail shells as shelters. This species contains normal and dwarf morphs at several localities. The normal morph inhabits rocky shorelines, whereas the dwarf morph invariably inhabits shell beds, where empty snail shells densely cover the lake bottom. Genetic evidence suggested that the dwarf morph evolved independently from the normal morph at two areas, and morphological analysis and evaluation of habitat structure revealed that the body sizes of morphs closely matched the available shelter sizes in their habitats. These findings suggest that the two morphs repeatedly evolved through divergent natural selection associated with the strategy for sheltering from predators.

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Hybridization and introgression are antagonistic to the process of speciation. If hybrids are viable and backcross, introgression will slow or prevent the evolution of reproductive isolation between populations. If species hybridize upon secondary contact, introgression will ultimately erode species boundaries. If hybridization is rampant and gene flow is high, the proportion of hybrids will approach values expected with random mating, and introgression will overcome species boundaries, homogenizing populations. Alternatively, if species are genetically distinct, the proportion of hybrids would be significantly less than expected with random mating, gene flow would be interrupted by selection against hybrids, and inviability or sterility of F 1 hybrids or backcrosses would be expected. Here, we investigate a system with characteristics expected by both of these opposing scenarios. Hybridization occurs between three species of reef fishes in the genus Hexagrammos at unexpected high frequencies in a zone of distributional overlap. Backcrossed individuals are detected, indicating F 1 hybrids are viable and capable of reproducing. Yet, these species are genetically distinct at multiple loci. To study this apparent paradox, we estimate the relative proportions of hybrids, patterns of symmetry, inviability, and cytonuclear disequilibria using one mitochondrial and two nuclear markers. We invoke selection against hybrids, at various life history stages, in the maintenance of species boundaries in this system.

Cooperatively breeding animals, in which helpers may participate in reproduction with dominant breeders, are ideal species for examining intraspecific variation in testis size because they often exhibit both monogamous breeding (low risk of sperm competition) and polyandrous breeding (high risk) within a population. However, little is known about testis investment as a result of sperm competition in these animals. The substrate-brooding cichlid fish Julidochromis ornatus has a cooperatively breeding system, in which some males mate monogamously and other males reproduce as dominant breeders or helpers within cooperatively breeding groups, in which male helpers frequently sire young. We examined the relationship between testis investment and male social status in relation to the risk of sperm competition. As predicted from sperm competition models, in groups with male helpers, both the male breeders and the male helpers invested more in testes mass, compared to breeding males without male helpers. We also found a positive relationship between the testes mass of male breeders and their male helpers, suggesting that males increase their investment in reproductive capability under the risk of sperm competition. Sperm competition models also predict that larger testes are associated with increased siring success. Our paternity analysis supported this prediction; we found a positive relationship between testis investment by male helpers and the number of offspring they sired.