mRNA variance has been proposed to play key roles in normal development, population fitness, adaptability, and disease. While variance in gene expression levels may be beneficial for certain cellular processes, for example in a cell’s ability to respond to external stimuli, variance may be detrimental for the development of some organs. In the bilaterally symmetric vertebrate limb buds, the amount of Sonic Hedgehog (SHH) protein present at specific stages of development is essential to ensure proper patterning of this structure. To our surprise, we found that SHH protein variance is present during the first 10 hr of limb development. The variance is virtually eliminated after the first 10 hr of limb development. By examining mutant animals, we determined that the ability of the limb bud apical ectodermal ridge (AER) to respond to SHH protein was required for reducing SHH variance during limb formation. One consequence of the failure to eliminate variance in SHH protein was the presence of polydactyly and an increase in digit length. These data suggest a potential novel mechanism in which alterations in SHH variance during evolution may have driven changes in limb patterning and digit length.

Limb patterning by Sonic hedgehog (Shh) is among the most highly touted and studied models of "morphogen" function 1 . Yet how Shh instructs distinct digit types (index to little finger) remains controversial. Both spatial concentration gradients 2,3 and temporal signal integration 4-6 have been proposed to explain how Shh patterns different digits, yet genetic studies in mouse suggested that Shh acts over a limited interval to specify digits 7 . Here, we replaced the cell survival function of Shh during limb bud outgrowth and demonstrate that a transient, early pulse of Shh activity is necessary and sufficient for normal limb development. Our lineage tracing of Shh response shows that Shh signals at very short-range during this time frame and patterns digits indirectly. We demonstrate that Gli3, the major Shh nuclear transducer 8,9 , is functionally unaltered and cryptic pathway re-activation doesn't occur. Our findings are incompatible with either spatial or temporal signal integration models and indicate Shh initiates a relay mechanism.Using a genetic test for relay signaling, we unexpectedly discovered that Shh is required indirectly to specify digit 1 (thumb), previously thought to be exclusively Shh-independent 10,11 . Our results uncover a unique digit 1 regulatory hierarchy, implicating Shh in digit 1 evolutionary adaptations, such as an opposable thumb. These findings illuminate Shh function in the related contexts of limb development, regeneration, and evolutionary adaptation, and lay the groundwork for elucidating how Shh triggers a relay network that becomes rapidly self-sustaining.

For centuries, students have been taught by passively listening to lectures, usually in large groups. In many fields, this type of course delivery is not ideal. In addition, students that have grown up with online access and/or have been forced to take online course during the recent pandemic, are seeking improved online educational experiences. As providers of education, Universities must adapt to this rapidly changing environment. The emergence of exciting, “disruptive” technologies has provided the tools needed for educators to revolutionize the delivery of higher education. The University of Florida, through UF Online, provides students the opportunity to enroll in fully online four-year undergraduate degrees. In this report, we describe the development of fully online laboratories for the upper-division course Evolutionary Developmental Biology, which fulfill the University of Florida’s laboratory requirement. This course is taken by Biology, Zoology, and related majors, including majors in the 100% online Biology degree. The experiments in the virtual laboratories would have been impossible to teach in a residential setting due to time constraints, cost, student safety issues, and regulatory/certification requirements. The ability to perform these experiments online provided University of Florida undergraduates located throughout the world access to unique, one-of-a-kind laboratory experiences. The virtual laboratories allowed students to undertake in-depth exploration and application of course content that is not possible in a residential setting. Completion of the Evolutionary Developmental Biology course provided undergraduate students the opportunity to obtain a science degree without ever visiting the university’s residential campus. During the Fall 2020 term, this course was made available to residential students, thus allowing this cohort of students to continue to make progress, 100% online, towards their undergraduate degree during the COVID19 pandemic.