Gonadotropin-induced changes in oviducal mRNA expression levels of sex steroid hormone receptors and activin-related signaling factors in the alligator.

Oviducts respond to hormonal cues from ovaries with tissue proliferation and differentiation in preparation of transporting and fostering gametes. These responses produce oviducal microenvironments conducive to reproductive success. Here, we investigated changes in circulating plasma sex steroid hormones concentrations and ovarian and oviducal mRNA expression to an in vivo gonadotropin (FSH) challenge in sexually immature, five-month-old alligators. Further, we investigated differences in these observed responses between alligators hatched from eggs collected at a heavily-polluted (Lake Apopka, FL) and minimally-polluted (Lake Woodruff, FL) site. In oviducts, we measured mRNA expression of estrogen, progesterone, and androgen receptors and also beta A and B subunits which homo- or heterodimerize to produce the transforming growth factor activin. In comparison, minimal inhibin alpha subunit mRNA expression suggests that these oviducts produce a primarily activin-dominated signaling milieu. Ovaries responded to a five-day FSH challenge with increased expression of steroidogenic enzyme mRNA which was concomitant with increased circulating sex steroid hormone concentrations. Oviducts in the FSH-challenged Lake Woodruff alligators increased mRNA expression of progesterone and androgen receptors, proliferating cell nuclear antigen, and the activin signaling antagonist follistatin. In contrast, Lake Apopka alligators displayed a diminished increase in ovarian CYP19A1 aromatase expression and no increase in oviducal AR expression, as compared to those observed in Lake Woodruff alligators. These results demonstrate that five-month-old female alligators display an endocrine-responsive ovarian-oviducal axis and environmental pollution exposure may alter these physiological responses.

Posthatching development of Alligator mississippiensis ovary and testis.

We investigated ovary and testis development of Alligator mississippiensis during the first 5 months posthatch. To better describe follicle assembly and seminiferous cord development, we used histochemical techniques to detect carbohydrate-rich extracellular matrix components in 1-week, 1-month, 3-month, and 5-month-old gonads. We found profound morphological changes in both ovary and testis. During this time, oogenesis progressed up to diplotene arrest and meiotic germ cells increasingly interacted with follicular cells. Concomitant with follicles becoming invested with full complements of granulosa cells, a periodic acid Schiff's (PAS)-positive basement membrane formed. As follicles enlarged and thecal layers were observed, basement membranes and thecal compartments gained periodic acid-methionine silver (PAMS)-reactive fibers. The ovarian medulla increased first PAS- and then PAMS reactivity as it fragmented into wide lacunae lined with low cuboidal to squamous epithelia. During this same period, testicular germ cells found along the tubule margins were observed progressing from spermatogonia to round spermatids located within the center of tubules. Accompanying this meiotic development, interstitial Leydig cell clusters become more visible and testicular capsules thickened. During the observed testis development, the thickening tunica albuginea and widening interstitial tissues showed increasing PAS- and PAMS reactivity. We observed putative intersex structures in both ovary and testis. On the coelomic aspect of testes were cell clusters with germ cell morphology and at the posterior end of ovaries, we observed "medullary rests" resembling immature testis cords. We hypothesize laboratory conditions accelerated gonad maturation due to optimum conditions, including nutrients and temperature. Laboratory alligators grew more rapidly and with increased body conditions compared with previous measured, field-caught animals. Additionally, we predict the morphological maturation observed in these gonads is concomitant with increased endocrine activities.

A novel preference for an invasive plant as a mechanism for animal hybrid speciation.

Homoploid hybrid speciation--speciation via hybridization without a change in chromosome number--is rarely documented and poorly understood in animals. In particular, the mechanisms by which animal homoploid hybrid species become ecologically and reproductively isolated from their parents are hypothetical and remain largely untested by experiments. For the many host-specific parasites that mate on their host, choosing the right host is the most important ecological and reproductive barrier between these species. One example of a host-specific parasite is the Lonicera fly, a population of tephritid fruit flies that evolved within the last 250 years likely by hybridization between two native Rhagoletis species following a host shift to invasive honeysuckle. We studied the host preference of the Lonicera fly and its putative parent species in laboratory experiments. The Lonicera fly prefers its new host, introduced honeysuckle, over the hosts of both parental species, demonstrating the rapid acquisition of preference for a new host as a means of behavioral isolation from the parent species. The parent taxa discriminate against each other's native hosts, but both accept honeysuckle fruit, leaving the potential for asymmetric gene flow from the parent species. Importantly, this pattern allows us to formulate hypotheses about the initial formation of the Lonicera fly. As mating partners from the two parent taxa are more likely to meet on invasive honeysuckle than on their respective native hosts, independent acceptance of honeysuckle by both parents likely preceded hybridization. We propose that invasive honeysuckle served as a catalyst for the local breakdown of reproductive isolation between the native parent species, a novel consequence of the introduction of an exotic weed. We describe behavioral mechanisms that explain the initial hybridization and subsequent reproductive isolation of the hybrid Lonicera fly. These results provide experimental support for a combination of host shift and hybridization as a model for hybrid speciation in parasitic animals.

Host shift to an invasive plant triggers rapid animal hybrid speciation.

Speciation in animals is almost always envisioned as the split of an existing lineage into an ancestral and a derived species. An alternative speciation route is homoploid hybrid speciation in which two ancestral taxa give rise to a third, derived, species by hybridization without a change in chromosome number. Although theoretically possible it has been regarded as rare and hence of little importance in animals. On the basis of molecular and chromosomal evidence, hybridization is the best explanation for the origin of a handful of extant diploid bisexual animal taxa. Here we report the first case in which hybridization between two host-specific animals (tephritid fruitflies) is clearly associated with the shift to a new resource. Such a hybrid host shift presents an ecologically robust scenario for animal hybrid speciation because it offers a potential mechanism for reproductive isolation through differential adaptation to a new ecological niche. The necessary conditions for this mechanism of speciation are common in parasitic animals, which represent much of animal diversity. The frequency of homoploid hybrid speciation in animals may therefore be higher than previously assumed.